A method of forming an optical sensor includes the following steps. A substrate is provided, and a read-out device is formed on the substrate. a first electrode electrically connected to the read-out device is formed on the substrate. a photosensitive silicon-rich dielectric layer is formed on the first electrode, wherein the photosensitive silicon-rich dielectric layer comprises a plurality of nanocrystalline silicon crystals. A second electrode is formed on the photosensitive silicon-rich dielectric layer.