A semiconductor device and a method for fabricating the semiconductor device are provided. In the method for fabricating the semiconductor device, at first, a dielectric layer is provided. Then, trenches are formed in the dielectric layer. Thereafter, the trenches are filled with spacer material to form a spacer structure in the dielectric layer for defining pixel regions. Then, lens structures are formed on the pixel regions. Each of the lens structures includes a first curved lens layer, a second curved lens layer and a curved color filter layer. The curved color filter layer is disposed on the second curved lens layer or between the first curved lens layer and the second curved lens layer.