The electronic structure of pure and Mg-doped wurtzite ZnO has been investigated by using first-principles ultrasoft pseudopotential method in the generalized gradient approximation. The calculation indicates that the band gap of ZnO broadens with increasing Mg-doping concentrations. Our work shows that the bottom of conduction band is determined by the Zn 4s electron states which can shift to a higher energy due to Mg-doping.