A CMOS image sensor and a method for fabricating the same are disclosed, in which the boundary between an active region and a field region is not damaged by ion implantation. The method for fabricating a CMOS image sensor includes forming a trench in a first conductive type semiconductor substrate, forming a first conductive type heavily doped impurity ion region in the semiconductor substrate at both sides of the trench, forming a device isolation film by interposing an insulating film between the trench and the device isolation, sequentially forming a gate insulating film and a gate electrode on the semiconductor substrate, and forming a second conductive type impurity ion region for a photodiode in the semiconductor substrate between the gate electrode and the device isolation film.