The electronic structures and optical properties of pure and Ga-doped wurtzite ZnO are studied by using first-principles plane wave pseudopotential method based on the density functional theory. The bonding of ZnO and changes in density of states are analyzed using of the crystal ligand field theory. Electron concentration is 2.42×1021 cm-3 by calculation, and carrier concentration of ZnO is raised 104 fold by doping Ga. Analysis of dielectric function, refractive index, absorption spectrum and reflectance spectrum of pure and Ga-doped ZnO shows that the optical absorption edge moving to high energy leads optical gap to broaden. In the visible light region, optical absorption coefficient and reflectivity are reduced and optical transmittance is increased significantly. Optical properties of ZnO are improved effectively by Ga doping.