Some embodiments of the present disclosure provide a semiconductor device. The semiconductor device includes a semiconductive substrate; a gate structure over a fin structure of the semiconductive substrate; a channel portion of the fin structure under the gate structure; and at least one epitaxy region disposed over the semiconductive substrate and in contact with the channel portion. The epitaxy region includes a substance with a first lattice constant larger than a second lattice constant of the semiconductive substrate; and a concentration profile of the substance in the epitaxy region being decreasing from near a bottom portion to near a top portion. The bottom portion is closer to the channel portion than the top portion.