A method for fabricating a semiconductor device includes the following steps. First, a first interlayer dielectric is formed on a substrate. Then, a gate electrode is formed on the substrate so that the periphery of the gate electrode is surrounded by the first interlayer dielectric. Afterwards, a patterned mask layer is formed on the gate electrode, and a bottom surface of the patterned mask layer is level with a top surface of the first interlayer dielectric. A spacer is then formed on each sidewall of the gate electrode. Subsequently, a second interlayer dielectric is formed to cover a top surface and each side surface of the patterned mask layer. Finally, a self-aligned contact structure is formed in the first interlayer dielectric and the second interlayer dielectric.