The electronic structures of pure and Mn-doped wurtzite ZnO have been calculated by first-principles ultra-soft pseudo-potential approach of the plane wave based upon the density functional theory, and the structure change, bandstructure, density of state and the difference charge density were studied. The calculated results were revealed that Mn-doping changed the band gap of the films, which increased with the increase of the Mn content. Moreover, the absorbence in strong UV absorption band of ZnO increased too.