Methods for forming an image sensor structure are provided. The method includes forming a light-sensing region in a substrate and forming a storage node adjacent to light-sensing region in the substrate. The method further includes forming a front side isolation structure partially surrounding an upper portion of the light-sensing region and forming a trench fully surrounding a bottom portion of the light-sensing region to expose a bottom surface of the front side isolations structure. The method further includes forming a backside isolation structure in the trench.