A motion detection method applied into an image sensor device includes: providing a plurality of regions of interest (ROIs) on at least one monitoring image; for each region of interest (ROI): detecting whether at least one motion event occurs within the each ROI; and determining a priority level of the each ROI according to at least one feature information of the at least one motion event; and determining an alarm schedule of the ROIs for a user according to a plurality of priority levels of the ROIs.