A CMOS image sensor and a manufacturing method thereof are disclosed. The gates of the transistors are formed in an active region of a unit pixel, and at the same time, a passivation layer is formed on an edge portion of the active region of a photodiode to have the same laminate structure as the gates of the transistors. Impurities for a diffusion region of the photodiode are ion-implanted into the active region for the photodiode, after the laminate structure is formed. The passivation layer prevents the edge portion from being damaged by ion implantation at the boundary or interface between the photodiode diffusion region and an isolation layer, which reduces dark current and/or leakage current of the CMOS image sensor.