A semiconductor device includes a substrate, a source/drain region, an etch stop layer, an oxide layer, an interlayer dielectric layer, and a contact plug. The source/drain region is in the substrate. The etch stop layer is over the source/drain region. The oxide layer is over the etch stop layer. The interlayer dielectric layer is over the oxide layer. The contact plug is electrically connected to the source/drain region through the interlayer dielectric layer, the oxide layer, and the etch stop layer.