The energy band structures, density of states, and optical properties of III A -doped wurtzite Mg 0.25 Zn 0.75 O (III A = Al, Ga, In) are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg 0.25 Zn 0.75 O:III A are larger than those of Mg 0.25 Zn 0.75 O because of the Burstein-Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg 0.25 Zn 0.75 O:III A are heavier than those of Mg 0.25 Zn 0.75 O, which is in agreement with the previous experimental result. The formation energies of MgZnO:Al and MgZnO:Ga are smaller than that of MgZnO:In, while their optical bandgaps are larger, so MgZnO:Al and MgZnO:Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet (UV) and deep UV optoelectronic devices.