A semiconductor device includes a buffer layer on a substrate, the buffer layer having a lattice constant different from that of the substrate, a fin structure upwardly protruding from the buffer layer, a gate electrode crossing over the fin structure, a cladding layer at a side of the fin structure and covering a top surface and sidewalls of the fin structure, and an interfacial layer between the cladding layer and the fin structure, the interfacial layer including a same element as the buffer layer.