A CMOS image sensor includes a pixel array unit, a row selection unit, and a logic circuit. The pixel array unit is used for sensing an object. The pixel array unit includes M pixels and P multiplexers and each of the M pixels is electrically connected to one of the P multiplexers, wherein M is a positive integer and P is a positive integer smaller than M. The row selection unit and the logic circuit are electrically connected to the P multiplexers. The row selection unit is used for generating a row selection signal. The logic circuit is used for determining a sensing region corresponding to the object wherein the sensing region includes N of the M pixels. Furthermore, the logic circuit controls Q multiplexers, which are electrically connected to the N pixels, to transmit the row selection signal to the N pixels.