A new external force field for active contour model, called anisotropic gradient vector flow, is presented to solve the problem that gradient vector flow (GVF) is difficult to enter the indentation. The diffusion term of GVF is the isotropic and highly smooth Laplacian operator with the same diffusion speed along tangent and normal directions. The diffusion of Laplacian operator is actually decomposed into the tangent and normal directions by the local image structures. Diffusion along the tangent direction enhances the edge, while diffusion along the normal direction removes noise and propagates the force field. This paper develops an anisotropic gradient vector flow based on the analysis of diffusion process of GVF along tangent and normal directions. In the proposed method, the diffusion speeds along the normal and tangent directions are adaptively obtained by the local structure of the image. The experimental results show that compared with GVF, the proposed method considering these two diffusion actions can enter long, thin indentation and improve the segmentation.