Abstract: In this paper, a new edge detection method——method using extremum of first directional derivative is proposed, based upon a general gradient edge model. Edge lines are extracted automatically from image, by acquiring points of extremum of first directional derivative vertical with edge line with Fourier transform in frequency domain. In detecting edges, non\|edge points are rejected using values of first directional derivatives vertical with edge line. Experiments involving both test and real image are conducted. Acquiring second directional derivative vertical with edge line is the main character of the method.