A method for fabricating an image sensor is described which includes forming an insulating layer on a semiconductor substrate and forming a recess in the semiconductor substrate and the insulating layer. An epitaxial structure is grown in the recess. A first polish treatment is then performed to the insulating layer and the epitaxial structure. The insulating layer is detected to obtain a signal intensity, and the signal intensity increases as a thickness of the insulating layer decreases. The first polish treatment stops when the signal intensity reaches a target value.