Disclosed are a CMOS image sensor and a manufacturing method thereof. The method includes the steps of: forming an isolation layer on a semiconductor substrate, defining an active region that includes a photo diode region and a transistor region; forming a gate in the transistor region, the gate including a gate electrode and a gate insulating layer; forming a first low-concentration diffusion region in the photo diode region; forming a second low-concentration diffusion region in the transistor region; forming a buffer layer over the substrate, the buffer layer covering the photo diode region; forming first and second insulating layers over the entire surface of the substrate, the first and second insulating layer having a different etching selectivity from each other; forming an insulating sidewall on sides of the gate electrode by selective removal of the second insulating layer; removing the first insulating layer from the transistor region; forming a high-concentration diffusion region in the exposed transistor region, partially overlapping the second low-concentration diffusion region; and forming a metal silicide layer on the high-concentration diffusion region.