Disclosed are a CMOS image sensor and a manufacturing method thereof. The present CMOS image sensor comprises: first, second, and third photo diodes and a plurality of transistors spaced at a predetermined distance in a semiconductor substrate; a diffusion blocking layer on substantially an entire surface of the substrate, including an opening therein exposing at least one of the photo diodes; an interlevel dielectric layer over the entire surface of the substrate, covering the diffusion blocking layer; first, second and third color filter layers over the interlevel dielectric layer, respectively corresponding to the first, second and third photo diodes, and a plurality of microlenses over the color filter layers, corresponding to each color filter layer.