A semiconductor device includes a substrate, a semiconductor fin, a gate stack, and an epitaxy structure. The semiconductor fin is disposed in the substrate. A portion of the semiconductor fin is protruded from the substrate. The gate stack is disposed over the portion of the semiconductor fin protruded from the substrate. The epitaxy structure is disposed on the substrate and adjacent to the gate stack. The epitaxy structure has a top surface facing away the substrate, and the top surface has at least one curved portion having a radius of curvature ranging from about 5 nm to about 20 nm.