A novel fast sub-pixel edge detection method for image measurement is proposed. Firstly, in order to find the edge point and edge direction at pixel level, the approximate positioning of the edge point is carried out by means of Sobel operator;secondly,the pixel is expanded along the edge direction of edge point to obtain the pixel grey vector with a 6 length. Then the accurate position of edge point can be extracted with a sub-pixel precision through the least square fit method. The experimental results show that the positioning accuracy amounts to 0.1 pixels and the running time of algorithm is 0.53s.