Tumblr has a video post feature where you can post a video that your followers can play directly from their dashboard. While this is convenient, occasionally it comes up where you want to embed a video along with some text, such as when reblogging someone's post or commenting on a note. To do this you'll need to copy the embedding code for the video you want to share -- available for most videos on YouTube -- and paste it into the body of your post using the HTML editor.
Open the YouTube video that you want to embed into your Tumblr post. Click the "Share" button below the video and click the "Embed" button below the video link.
Change the embedding settings. You can choose a height and width from the defaults, or specify your own; when you enter one size the other will be chosen to keep the video in proportion. The minimum width is 200 pixels.
Copy the embed code.
Open a new text post in Tumblr and click the "HTML" button on the formatting toolbar. This will open the HTML editor in a pop-up window.
Paste the embed code into the HTML editor and click "Update." The video will show up in your rich text editor. Click "Create Post" to finish the post.
Unlike the video posts users will not be able to view your video on the dashboard. Instead your video will show up as a little grey box that users can click to open the video on your Tumblr page.
Not all YouTube videos have an embedding code; some users disable the embedding code on their videos.