The conduction band structure of GaAs nanowires with various cross-sectional shapes and orientations was calculated by a tight-binding model and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases. However, the amount of the increase in mass is strongly dependent on the wire orientations and substrate faces of nanowires, which originates from the anisotropy of Γ valley of bulk GaAs. © 2013 IEEE.