Provided is a semiconductor device including a substrate, a gate structure, a second dielectric layer and a source/drain region. A first dielectric layer is disposed on the substrate, and the first dielectric layer has a trench therein. The gate structure is disposed on the substrate in the trench and includes a work function metal layer and a metal layer. The work function metal layer is disposed in the trench, and includes a TiAl3 phase metal layer. A height of the work function metal layer disposed on a sidewall of the trench is lower than a height of a top surface of the first dielectric layer. The metal layer fills the trench. The second dielectric layer is disposed between the gate structure and the substrate. The source/drain region is disposed in the substrate at two sides of the gate structure.