Youtube has not provided the facility to download the videos directly.We can use tubemate app to download the videos.TubeMate,YouTube down loader enables you to quickly access, search, share, and download Youtube videos. Because downloading always happens in the background.We need to install the tubemate app mobile and this will available in play store as free app
Open the tubemate app which displays some videos by default ,search and select the videos which you need to download.
Click on the green arrow mark and videos will be downloaded to your mobile.Also we can specify the format during the download.