A new spatio-temporal pre-filter based on anisotropic diffusion is proposed. The filter has a nonlinear feature; it is capable of smoothing out white noise and insignificant textures in a video sequence in order to improve coding efficiency, while preserving such significant information as object edges. Computer simulation results verify that the subjective quality of a compressed video sequence is usually better when the sequence is preprocessed by the proposed filter, because the blocking artifacts which otherwise appear in the decoded images are alleviated.