A semiconductor device includes: a substrate having a first semiconductor layer, an insulating layer, and a second semiconductor layer; an active device on the substrate; an interlayer dielectric (ILD) layer on the active device; a first contact plug adjacent to the active device; and a second contact plug in the ILD layer and electrically connected to the active device. Preferably, the first contact plug includes a first portion in the insulating layer and the second semiconductor layer and a second portion in the ILD layer, in which a width of the second portion is greater than a width of the first portion.