Abstract: The paper analyzes the existent question about the differential algorithm for image segmentation first, and then proposes a differential extremum based edge detection algorithm. The feature of the algorithm lies in that the edge pixel is directly extracted at a local window for the edge detection, the extracted edge is a true edge, and the edge possesses good continuity. The algorithm is suited for-segmentation of the image having inhomogeneous-illumination. The segmentation experiment gaves a satisfactory result.