The invention provides an image sensor and a fabricating method thereof. A pixel area and a peripheral circuit area can have a step difference on a semiconductor substrate. A Complimentary Metal Oxide Semiconductor (CMOS) circuit can be provided on the pixel area, and an interlayer dielectric layer can be provided on the pixel area and the peripheral circuit area. A photodiode can be provided on the interlayer dielectric layer of the pixel area such that the top of the photodiode, or an intrinsic layer of the photodiode, is about even with the top of the interlayer dielectric layer of the peripheral circuit area. The invention is capable of forming the photodiode having PIN structure on the semiconductor substrate, and enabling the transistor circuit to be integrated with the photodiode vertically; therefore, the fill factor is approaches 100%; with the same pixel size, the invention is capable of providing sensitivity higher than that provided by current technique.