An image sensor device is provided. The image sensor device includes a semiconductor substrate having a first side, a second side opposite to the first side, and at least one light-sensing region close to the first side. The image sensor device includes a dielectric feature covering the second side and extending into the semiconductor substrate. The dielectric feature in the semiconductor substrate surrounds the light-sensing region. The image sensor device includes a reflective layer in the dielectric feature in the semiconductor substrate, wherein a top portion of the reflective layer protrudes away from the second side, and a top surface of the reflective layer and a top surface of the insulating layer are substantially coplanar.