An image sensor is disclosed. The image sensor includes an epitaxial layer, a plurality of plug structures and an interconnect structure. Wherein the plurality of plug structures are formed in the epitaxial layer, and each plug structure has doped sidewalls, the epitaxial layer and the doped sidewalls form a plurality of photodiodes, the plurality of plug structures are used to separate adjacent photodiodes, and the epitaxial layer and the doped sidewalls are coupled to the interconnect structure via the plug structures. An associated method of fabricating the image sensor is also disclosed. The method includes: providing a substrate having a first-type doped epitaxial substrate layer on a second-type doped epitaxial substrate layer; forming a plurality of isolation trenches in the first-type doped epitaxial substrate layer; forming a second-type doped region along sidewalls and bottoms of the plurality of isolation trenches; and filling the plurality of isolation trenches by depositing metal.