A method for fabricating a metal-oxide-semiconductor field-effect transistor includes the following steps. Firstly, a substrate is provided. A gate structure, a first spacer, a second spacer and a source/drain structure are formed over the substrate. The second spacer includes an inner layer and an outer layer. Then, a thinning process is performed to reduce the thickness of the second spacer, thereby retaining the inner layer of the second spacer. After a stress film is formed on the inner layer of the second spacer and the source/drain structure, an annealing process is performed. Afterwards, the stress film is removed.