A method for adjusting a contrast value of a video capturing end to a optimum contrast value is provided. Firstly, a first video image and a first contrast value of the first video image are gathered by a software part. A reference image is generated according to the first video image. The first video image is compared with the reference image to generate a first image evaluation value and an operation is performed to determine whether the first image evaluation value is greater than a first threshold. When the first image evaluation value is smaller than the first threshold, a second contrast value is generated according to the first contrast value and a dynamical contrast adjusting value. The second contrast value is sent to a firmware part by the software part to adjust a contrast value of the video capturing end according to the second contrast value.