A semiconductor device includes channel layers disposed over a substrate, a source/drain region disposed over the substrate, a gate dielectric layer disposed on and wrapping each of the channel layers, and a gate electrode layer disposed on the gate dielectric layer and wrapping each of the channel layers. Each of the channel layers includes a semiconductor wire made of a core region, and one or more shell regions. The core region has an approximately square-shape cross section and a first shell of the one or more shells forms a first shell region of an approximately rhombus-shape cross section around the core region and is connected to an adjacent first shell region corresponding to a neighboring semiconductor wire.