A method for forming an image sensor device is provided. The method includes forming a first trench in a semiconductor substrate. The semiconductor substrate has a front surface and a back surface, and the first trench extends from the front surface into the semiconductor substrate. The method includes forming a first isolation structure in the first trench. The method includes forming a light-sensing region in the semiconductor substrate. The first isolation structure surrounds the light-sensing region. The method includes forming a second trench in the semiconductor substrate. The second trench extends from the back surface into the semiconductor substrate and exposes the first isolation structure. The method includes forming a second isolation structure in the second trench. The second isolation structure includes a light-blocking structure to absorb or reflect incident light.