Image sensor structures are provided. The image sensor structure includes a substrate having a front side and a backside and a light-sensing region formed in the substrate. The image sensor structure further includes a front side isolation structure surrounding the light sensing region and having an opening region in a top view and a backside isolation structure formed at the backside of the substrate and encompassing the light-sensing region and vertically overlapping the opening region. The image sensor structure further includes a first gate structure formed over the front side of the substrate and overlapping the opening region and the front side isolation structure and a storage node in the substrate adjacent to the first gate structure. In addition, the storage node extends into the opening region.