A semiconductor device includes a gate stack, an isolation structure and a strained feature. The gate stack is over a substrate. The isolation structure is in the substrate. The strained feature is disposed between the gate stack and the isolation structure and disposed in the substrate. The strained feature includes an upper surface adjacent to the isolation structure having a first crystal plane and a sidewall surface adjacent to the gate stack having a second crystal plane. The first crystal plane is different from the second crystal plane.