A CMOS image sensor according to the present invention has a low-voltage photodiode which is fully depleted at a bias of 1.2-4.5V. The photodiode comprises: a P-epi layer; a field oxide layer dividing the P-epi layer into a field region and an active region; a N− region formed within the P-epi layer, wherein the first impurity region is apart from the isolation layer; and a P0 region of the conductive type formed beneath a surface of the P-epi layer and on the N− region, wherein a width of the P0 region is wider than that of the N− region so that a portion of the P0 region is formed on the P-epi layer, whereby the P0 region has the same potential as the P-epi layer.