A CMOS image sensor and manufacturing method thereof are disclosed. The present CMOS image sensor comprises: a semiconductor substrate including an active region having a photo diode region and a transistor region; a gate on the active region, comprising a gate insulating layer and a gate electrode thereon; a first source/drain diffusion region in the transistor region at one side of the gate electrode, including a first conductivity type dopant; a second photo diode diffusion region in the region at the other side of the gate electrode, the second diffusion region including a first conductivity type dopant; insulating sidewalls on sides of the gate electrode; and a third diffusion region over or in the second diffusion region, extending below one of the insulating sidewalls (e.g., closest to the photo diode region), and including a second conductivity type dopant.