A method forming an image sensor includes: providing a substrate including a plurality of sensing portions; forming a color filter layer on the substrate; forming a micro-lens material layer on the color filter layer; and forming a hard mask pattern on the micro-lens material layer, wherein the hard mask pattern has a first gap and a second gap larger than the first gap. The method includes reflowing the hard mask pattern into a plurality of dome shapes; transferring the plurality of dome shapes into the micro-lens material layer to form a plurality of micro-lenses; and forming a top film conformally on the plurality of micro-lenses.