The invention provides a method for determining a coordinate on a small bitmap by using the motion vector, comprising the following steps: calculating a coordinate which currently corresponds to the center of the picture of on optical scanning device according to the small bitmap decoding mode when the user holds the optical scanning device to scan the small bitmap on the displaying media; calculating the corresponding motion vector according to the difference of different pictures which are scanned by the optical scanning device at different time point; and calculating the position of the center of the picture on the displaying media at any time point. Therefore, the huge computation amount resulted from the frequent decoding can be avoided due to the small computation amount of the motion vector. The error resulted from the vibration of the displaying media can be corrected immediately through further decoding the coordinate in the center of the picture in the changeless or changeable time interval, and the position of the center of the picture on the displaying media, which is calculated at any time, can not be subjected to the accumulated error.