diff --git a/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel+Compress.swift b/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel+Compress.swift
new file mode 100644
index 000000000..e9c1df676
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel+Compress.swift
@@ -0,0 +1,33 @@
+//
+//  AttachmentViewModel+Compress.swift
+//  
+//
+//  Created by MainasuK on 2022/11/11.
+//
+
+import UIKit
+import AVKit
+
+extension AttachmentViewModel {
+    func comporessVideo(url: URL) async throws -> URL {
+        let task = Task { () -> URL in
+            let urlAsset = AVURLAsset(url: url)
+            guard let exportSession = AVAssetExportSession(asset: urlAsset, presetName: AVAssetExportPresetMediumQuality) else {
+                throw AttachmentError.invalidAttachmentType
+            }
+            let outputURL = try FileManager.default.createTemporaryFileURL(
+                filename: UUID().uuidString,
+                pathExtension: url.pathExtension
+            )
+            exportSession.outputURL = outputURL
+            exportSession.outputFileType = AVFileType.mp4
+            exportSession.shouldOptimizeForNetworkUse = true
+            await exportSession.export()
+            return outputURL
+        }
+        
+        self.compressVideoTask = task
+        
+        return try await task.value
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel.swift b/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel.swift
index 9409e7380..7bbaaefaf 100644
--- a/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel.swift
+++ b/MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/Attachment/AttachmentViewModel.swift
@@ -43,6 +43,8 @@ final public class AttachmentViewModel: NSObject, ObservableObject, Identifiable
     @Published var caption = ""
     @Published var sizeLimit = SizeLimit()
     
+    var compressVideoTask: Task<URL, Error>?
+    
     // output
     @Published public private(set) var output: Output?
     @Published public private(set) var thumbnail: UIImage?      // original size image thumbnail
@@ -120,9 +122,20 @@ final public class AttachmentViewModel: NSObject, ObservableObject, Identifiable
         defer {
             Task { @MainActor in
                 do {
-                    let output = try await load(input: input)
-                    self.output = output
+                    var output = try await load(input: input)
+                    
+                    switch output {
+                    case .video(let fileURL, let mimeType):
+                        let compressedFileURL = try await comporessVideo(url: fileURL)
+                        output = .video(compressedFileURL, mimeType: mimeType)
+                        try? FileManager.default.removeItem(at: fileURL)    // remove old file
+                    default:
+                        break
+                    }
+                    
                     self.outputSizeInByte = output.asAttachment.sizeInByte.flatMap { Int64($0) } ?? 0
+                    self.output = output
+                    
                     self.update(uploadState: .ready)
                     self.delegate?.attachmentViewModel(self, uploadStateValueDidChange: self.uploadState)
                 } catch {