A video object segmentation method based on MAP (maximum a posteriori) of pixel blocks is proposed. Firstly

a reference frame is built from three continuous frames and a pixel block-based difference image is calculated. Secondly

a pixel block-based difference image is described by Gibbs random field model

the moving object blocks are detected by MAP. Last

the contour of the moving object is extracted by the snake method. The experiments results show that the algorithm can avoid the losing and mistakes with a good speed.