In order to play YouTube video in QuickTime player, you need to download and convert YouTube video to QuickTime .mov format first. It is very easy and fast to download YouTube video and convert YouTube video to QuickTime .mov using Clone2Go Video Converter for Mac.
Download YouTube Video
Download the latest version of Clone2Go Video Converter for Mac, install and run it.
Click the icon in the interface of the program. Then you can start to download and convert YouTube videos in the pop-up "Clone2Go YouTube Downloader" window.
Select output format: "QuickTime Movie (*.mov)"
After the downloaded videos are imported into Clone2Go Video Converter Professional, go to the "Output" panel. Click the "Profile" drop-down list to choose "QuickTime Movie (*.mov)" as the output format.
When all settings are done, click the "Convert Now" button to start convert YouTube to QuickTime .mov format.