1. A video device for realtime pedaling frequency estimation, mounted on a bike and comprising an image capture unit capturing continuous dynamic images of an upper body of a biker; an image recognition unit electrically connected with said image capture unit and recognizing images of symmetric regions of said biker and images of swings of said symmetric regions from said continuous dynamic images; a microprocessor electrically connected with said image recognition unit, calculating a frequency of periodical swings of said biker from said images of said symmetric regions and said images of said swings of said symmetric regions, and then obtaining a pedaling frequency; and a display device electrically connected with said microprocessor and presenting information of said pedaling frequency.