A method for forming an image sensor device is provided. The method includes providing a substrate. The substrate has a front surface and a back surface, and the substrate has a light-receiving region and a device region. The method includes forming a first transistor and a first source/drain structure respectively in the light-receiving region and the device region. The first transistor includes a first gate structure, a light-sensing structure, a second source/drain structure, the first gate structure is over the front surface, the light-sensing structure and the second source/drain structure are formed in the substrate and are respectively located at opposite first sides of the first gate structure, the first source/drain structure is formed in the substrate, and the first source/drain structure is electrically connected to the second source/drain structure. The method includes forming a light-blocking layer over the back surface.