A PMOS field effect transistor includes a substrate, a first nitride layer, a mesa structure, two gate oxide films, a gate stack layer and a second nitride layer. The substrate has a oxide layer and a first doping area. The first nitride layer is located on the oxide layer. The mesa structure includes a first strained Si—Ge layer, an epitaxial Si layer and a second strained Si—Ge layer. The first strained Si—Ge layer is located on the oxide layer and the first nitride layer. The epitaxial Si layer is located on the first strained Si—Ge layer. The second strained Si—Ge layer is located on the epitaxial Si layer. In the surface layer of the second strained Si—Ge layer, there is a second doping area. The two gate oxide films are located at two sides of the mesa structure.