A semiconductor device includes a substrate, a gate structure on the substrate, a first etch stop layer, a second etch stop layer, and an interlayer insulation layer that are stacked on the gate structure, and a contact plug penetrating the interlayer insulation layer, the second etch stop layer, and the first etch stop layer and contacting a sidewall of the gate structure. The contact plug includes a lower portion having a first width and an upper portion having a second width. A lower surface of the contact plug has a stepped shape.