An image sensor and a method for fabricating the image sensor are provided. The method for fabricating the image sensor includes forming a first insulating layer on a semiconductor epitaxial layer having multiple pixel regions; patterning a portion of the semiconductor epitaxial layer and the first insulating layer in a boundary region between the pixel regions to form a trench; forming a buried insulating layer on the first insulating layer, filling the trench, the buried insulating layer having a planar top surface; forming a second insulating layer on the buried insulating layer; forming a first mask pattern on the second insulating layer, the first mask pattern defining an opening overlapping the trench; and performing an ion implantation process using the first mask pattern as an ion implantation mask to form a first type potential barrier region in a bottom of the trench.