In this paper a method of image interpolation based on bidirectional diffusion is proposed. With this method, the edge width of interpolated images is effectively reduced and crisp and smooth edges are obtained. In this bidirectional diffusion, forward diffusion occurs in brighter lateral on edge ramp and backward diffusion proceeds in darker lateral. And the intensity of diffusion is adjusted adaptively according to the image features, which avoid the appearance of artifacts and false textures in the interpolated image. Numerical experiments on real images show that images interpolated with the proposed method have smaller edge width and are almost artifact-free.