An image sensor includes a substrate, a photosensitive unit in the substrate, a dielectric grid over the substrate, and a color filter over the photosensitive unit and surrounded by the dielectric grid. The dielectric grid has a first portion and a second portion over the first portion, and the second portion of the dielectric grid has a rounded top surface extending upwards from a sidewall of the first portion of the dielectric grid. The color filter has a first portion lower than a lowermost portion of the rounded top surface of the second portion of the dielectric grid and a second portion higher than the lowermost portion of the rounded top surface of the second portion of the dielectric grid.