Abstract: Shadowed area will appear in the displayed image when the field of view of a surveillance system is obscured, and it influences the analysis and further processing of the image. A real time shadow detection algorithm is presented to solve this problem. Firstly, the image is divided into image blocks based on inter-frame movement feature of sequence images in this algorithm. Secondly, the algorithm judges if this image frame is obscured and detects the possible shadowed area with the inter-frame predictor based on the gray mean of the image block. Finally, the shadow area is confirmed with a space domain filter for eliminating the erroneous detection areas. The experiment shows that this algorithm is real time and feasible.