This paper presents a modified background subtraction algorithm through frame prepro-cessing using smoothing with RGB data format to remove noise, unwanted objects( rain and clouds ) and to get more information from the frame then the preprocessed frames are used for background modeling to provide a statistical description of the entire back-ground scene using non-recursive frame differencing technique with 4 frames difference and scaled frame to minimize the modeling time to extract foreground objects using 2 pre-calculated thresholds(strong threshold and weak threshold)not only one threshold to give an accurate decision about moving objects in the video sequence. This modified algorithm decreased the false alarm rate by 25% to 30% and faster pro-cessing time comparing with normal background subtraction algorithm in outdoor motion detection system.