diff --git a/tibok/tibok/Models/Document.swift b/tibok/tibok/Models/Document.swift
index 68be239..a3932f1 100644
--- a/tibok/tibok/Models/Document.swift
+++ b/tibok/tibok/Models/Document.swift
@@ -73,4 +73,18 @@ struct Document: Identifiable, Equatable {
     var lineCount: Int {
         content.components(separatedBy: .newlines).count
     }
+    
+    /// Whether the document can be previewed as markdown
+    /// Only markdown files and untitled documents can be previewed
+    var isPreviewSupported: Bool {
+        // If no file URL, it's a new untitled document - support preview
+        guard let fileURL = fileURL else {
+            return true
+        }
+        
+        // Check file extension
+        let ext = fileURL.pathExtension.lowercased()
+        let supportedExtensions = ["md", "markdown", "mdown", "mkd"]
+        return supportedExtensions.contains(ext)
+    }
 }
diff --git a/tibok/tibok/Views/PreviewView.swift b/tibok/tibok/Views/PreviewView.swift
index a33006e..a383a20 100644
--- a/tibok/tibok/Views/PreviewView.swift
+++ b/tibok/tibok/Views/PreviewView.swift
@@ -100,6 +100,9 @@ struct PreviewView: View {
             if appState.currentDocument.isEmpty {
                 // Empty state
                 PreviewEmptyStateView()
+            } else if !appState.currentDocument.isPreviewSupported {
+                // Unsupported file type
+                PreviewUnsupportedView(fileExtension: appState.currentDocument.fileURL?.pathExtension ?? "")
             } else {
                 // Preview content using WKWebView
                 MarkdownWebView(html: renderedHTML, baseURL: resourceBaseURL)
@@ -429,3 +432,43 @@ struct PreviewEmptyStateView: View {
         .frame(maxWidth: .infinity, maxHeight: .infinity)
     }
 }
+
+// MARK: - Preview Unsupported State
+
+struct PreviewUnsupportedView: View {
+    let fileExtension: String
+    
+    var body: some View {
+        VStack(spacing: 16) {
+            Spacer()
+            
+            Image(systemName: "doc.text")
+                .font(.system(size: 40))
+                .foregroundColor(.secondary.opacity(0.4))
+            
+            Text("Preview Not Available")
+                .font(.headline)
+                .foregroundColor(.secondary)
+            
+            Text("Preview is only available for Markdown files")
+                .font(.subheadline)
+                .foregroundColor(.secondary.opacity(0.7))
+                .multilineTextAlignment(.center)
+            
+            if !fileExtension.isEmpty {
+                Text("Current file type: .\(fileExtension)")
+                    .font(.system(size: 11))
+                    .foregroundColor(.secondary.opacity(0.5))
+                    .padding(.top, 4)
+            }
+            
+            Text("⌘\\ to toggle preview")
+                .font(.system(size: 11))
+                .foregroundColor(.secondary.opacity(0.5))
+                .padding(.top, 4)
+            
+            Spacer()
+        }
+        .frame(maxWidth: .infinity, maxHeight: .infinity)
+    }
+}
diff --git a/tibok/tibokTests/DocumentTests.swift b/tibok/tibokTests/DocumentTests.swift
index 7971563..e23acd5 100644
--- a/tibok/tibokTests/DocumentTests.swift
+++ b/tibok/tibokTests/DocumentTests.swift
@@ -50,4 +50,68 @@ struct DocumentTests {
         let doc = Document(content: "Just one line")
         #expect(doc.lineCount == 1)
     }
+    
+    @Test("Preview is supported for markdown files")
+    func previewIsSupportedForMarkdownFiles() {
+        let mdURL = URL(fileURLWithPath: "/path/to/file.md")
+        let doc = Document(fileURL: mdURL)
+        #expect(doc.isPreviewSupported == true)
+    }
+    
+    @Test("Preview is supported for all markdown extensions")
+    func previewIsSupportedForAllMarkdownExtensions() {
+        let extensions = ["md", "markdown", "mdown", "mkd"]
+        for ext in extensions {
+            let url = URL(fileURLWithPath: "/path/to/file.\(ext)")
+            let doc = Document(fileURL: url)
+            #expect(doc.isPreviewSupported == true, ".\(ext) files should be previewable")
+        }
+    }
+    
+    @Test("Preview is not supported for non-markdown files")
+    func previewIsNotSupportedForNonMarkdownFiles() {
+        let tsxURL = URL(fileURLWithPath: "/path/to/file.tsx")
+        let doc = Document(fileURL: tsxURL)
+        #expect(doc.isPreviewSupported == false)
+    }
+    
+    @Test("Preview is not supported for common text files")
+    func previewIsNotSupportedForCommonTextFiles() {
+        let extensions = ["txt", "tsx", "jsx", "js", "ts", "py", "swift"]
+        for ext in extensions {
+            let url = URL(fileURLWithPath: "/path/to/file.\(ext)")
+            let doc = Document(fileURL: url)
+            #expect(doc.isPreviewSupported == false, ".\(ext) files should not be previewable")
+        }
+    }
+    
+    @Test("Preview is supported for untitled documents")
+    func previewIsSupportedForUntitledDocuments() {
+        let doc = Document.new()
+        #expect(doc.isPreviewSupported == true)
+    }
+    
+    @Test("Preview support is case insensitive")
+    func previewSupportIsCaseInsensitive() {
+        let upperURL = URL(fileURLWithPath: "/path/to/file.MD")
+        let mixedURL = URL(fileURLWithPath: "/path/to/file.Markdown")
+        
+        let upperDoc = Document(fileURL: upperURL)
+        let mixedDoc = Document(fileURL: mixedURL)
+        
+        #expect(upperDoc.isPreviewSupported == true)
+        #expect(mixedDoc.isPreviewSupported == true)
+    }
+    
+    @Test("Preview is not supported for files without extensions")
+    func previewIsNotSupportedForFilesWithoutExtensions() {
+        let readmeURL = URL(fileURLWithPath: "/path/to/README")
+        let licenseURL = URL(fileURLWithPath: "/path/to/LICENSE")
+        
+        let readmeDoc = Document(fileURL: readmeURL)
+        let licenseDoc = Document(fileURL: licenseURL)
+        
+        #expect(readmeDoc.isPreviewSupported == false)
+        #expect(licenseDoc.isPreviewSupported == false)
+    }
 }
diff --git a/tibok/user_docs/FAQ.md b/tibok/user_docs/FAQ.md
index 024ed93..e656e44 100644
--- a/tibok/user_docs/FAQ.md
+++ b/tibok/user_docs/FAQ.md
@@ -92,6 +92,9 @@ Use View > Toggle Preview or press Cmd+\.
 ### Does the preview update in real-time?
 Yes, the preview updates as you type.
 
+### Why does the preview show "Preview Not Available"?
+Preview is only available for Markdown files (`.md`, `.markdown`, `.mdown`, `.mkd`). When you open other file types (like `.tsx`, `.txt`, `.js`), the preview pane will display a message indicating that preview is not supported for that file type. You can hide the preview with Cmd+\ to maximize your editor space.
+
 ### How do I create nested lists?
 Use 2-space indentation per nesting level:
 ```markdown
diff --git a/tibok/user_docs/features/preview.md b/tibok/user_docs/features/preview.md
index da7581f..2ddfc9b 100644
--- a/tibok/user_docs/features/preview.md
+++ b/tibok/user_docs/features/preview.md
@@ -2,6 +2,16 @@
 
 tibok provides a live preview pane that renders your markdown as you type.
 
+## File Type Support
+
+Preview is **only available for Markdown files** with the following extensions:
+- `.md`
+- `.markdown`
+- `.mdown`
+- `.mkd`
+
+When you open a non-markdown file (e.g., `.tsx`, `.txt`, `.js`), the preview pane will display a message indicating that preview is not available for that file type. You can toggle the preview pane off with `Cmd+\` to maximize your editor space.
+
 ## Showing/Hiding Preview
 
 - **Menu**: View > Toggle Preview