A semiconductor device may include a substrate, a first nanowire, a second nanowire, a first gate insulating layer, a second gate insulating layer, a first metal layer and a second metal layer. The first gate insulating layer may be along a periphery of the first nanowire. The second gate insulating layer may be along a periphery of the second nanowire. The first metal layer may be on a top surface of the first gate insulating layer along the periphery of the first nanowire. The first metal layer may have a first crystal grain size. The second metal layer may be on a top surface of the second gate insulating layer along the periphery of the second nanowire. The second metal layer may have a second crystal grain size different from the first crystal grain size.