A method of fabricating an MOS device is provided. First, gates and source/drain regions of transistors are formed on a substrate. A photodiode doped region and a floating node doped region are formed in the substrate. Thereafter, a spacer stacked layer including a bottom layer, an inter-layer and a top layer is formed to cover each gate of the transistors. Afterwards, a first mask layer having an opening exposing at least the photodiode doped region is formed on the substrate, and then the top layer exposed by the opening is removed. Next, the first mask layer is removed, and then a second mask layer is formed on a region correspondingly exposed by the opening. A portion of the top layer and the inter-layer exposed by the second mask layer is removed to form spacers on sidewalls of the gates.