A semiconductor structure is disclosed, including a substrate having therein a first well of a first conductivity type and a second well of a second conductivity type, a first MOS transistor of the first conductivity type and a second MOS transistor of the second conductivity type. The first MOS transistor is disposed on the second well, including a gate structure on the second well and a strained layer of the first conductivity type in an opening in the second well beside the gate structure. The difference between the cell parameter of a portion of the strained layer near the bottom of the opening and that of the substrate is less than the difference between the cell parameter of a portion of the strained layer apart from the bottom of the opening and that of the substrate. The second MOS transistor is disposed on the first well.