A metal-oxide-semiconductor (MOS) transistor device is provided. The MOS transistor device includes a substrate, a gate structure, a spacer, a source/drain region and a barrier layer. The gate structure is disposed on the substrate. The gate structure includes a gate and a gate dielectric layer disposed between the gate and the substrate. The spacer is disposed on the sidewall of the gate structure. The source/drain region is disposed in the substrate on two sides of the spacer. The barrier layer is disposed around the source/drain region. The source/drain region and the barrier layer are fabricated using an identical material. However, the doping concentration of the source/drain region is larger than the doping concentration of the barrier layer.