Classic Sobel algorithm is commonly used in edge detection. However, it has detected a lot of noise points and created broken license plate characters, which increased the difficulty of extracting license plate and characters, recognition, and reduced the accuracy of license plate location. In this paper, Sobel algorithm has been improved for license plate recognition system. The improved Sobel algorithm has achieved good results of license plate location and it has good anti-interference capability. And it can maintain the integrity of the characters to a large extent.