A semiconductor structure including a semiconductor substrate, a first patterned dielectric layer, a grating coupler and a waveguide is provided. The semiconductor substrate includes an optical reflective layer. The first patterned dielectric layer is disposed on the semiconductor substrate and covers a portion of the optical reflective layer. The grating coupler and the waveguide are disposed on the first patterned dielectric layer, wherein the grating coupler and the waveguide are located over the optical reflective layer.