Abstract: Although median filter has been widely used in seismic data processing, especially in processing of VSP data due to its many merits, it is difficult to understand the behaviors of the median filter, because it is one kind of nonlinear filter with zero response to spike pulse, and has no "actual" amplitude and phase spectrum. However, the effects of the filter can still be analyzed via many straightforward numerical means. Among them analysis of its responses to different signals is an effective and practical way to study the behaviors of the filter. Some characteristics of the median filter are presented in the paper.