A method for manufacturing a BSI image sensor includes following steps: A substrate is provided. The substrate includes a front side and a back side opposite to the front side. The substrate further includes a plurality of isolation structures and a plurality of sensing elements formed therein. Next, the isolation structures are exposed from the back side of the substrate. Subsequently, a thermal treatment is performed to the back side of the substrate to form a plurality of cambered surfaces on the back side of the substrate. The cambered surfaces are formed correspondingly to the sensing elements, respectively.