You may do it easily using converter TuneCab.
TuneCab is media converter for DRM-protected or unprotected files for playing on PCs, mobile phones, MP3 players, MP4 players and iPods, PDAs, PSPs and other devices. It converts music, films, video clips, audio books to commonly used formats such as MP3, AAC, WAV audio, MPEG4, DivX movie etc, has YouTube Ripper and CD Ripper.
How to use YouTube Ripper:
Open YouTube video in Internet Explorer -> Tools -> Rip YouTube file embedded in this page.
The video will be downloaded automatically and converted by TuneCab Ultra to desired format (video or audio). Then transfer it to you player and ENJOY!

I lie this youtube downloader.

Why not try this Youtube download and conversion software, it works well.
http://www.youtubetoipod.org/

New edition - TuneCab Online Video Catcher. Download any online video.