Provided is a CMOS (complementary metal oxide semiconductor) image sensor and a manufacturing method thereof. In the method, a photodiode, an interlayer insulating layer, a color filter layer, and a planarizing layer are sequentially formed on a substrate. A photoresist is applied on the planarizing layer. The photoresist is selectively patterned to form a plurality of photoresist patterns. A surface of each photoresist is hardened. The hardened photoresist patterns are reflowed to form microlenses.