a method for fabricating an image sensor is provided . a substrate is provided , and then a plurality of photoresist patterns is formed on the substrate . the photoresist patterns are arranged in a first array and defined by a plurality of photomask patterns arranged as a photomask pattern array , wherein a top view of each photoresist pattern has a substantially square shape and a distance between two neighboring photoresist patterns decreases from a center of the first array toward an edge of the first array . besides , each photomask pattern includes a transparent portion and an opaque portion , wherein an area proportion of the transparent portion included in a photomask pattern increases from the center toward the edge of the photomask pattern array . then , a thermal reflow step is performed to convert the photoresist patterns into a plurality of microlenses arranged in a second array .