Low resistance p-type ZnO thin films were prepared by in situ oxidation of Zn3N2 films which were deposited by reactive radio-frequency magnetron sputtering of zinc in Ar-N2 mixture atmosphere. The effects of oxidation temperature and time on structural, electronic and optical properties of the samples were investigated by X-ray diffraction, scanning electron microscopy, Hall-effect measurements, transmittance spectra and photoluminescence spectra. It was found that the sample oxidized at 450 ℃ contains Zn2N3 besides ZnO. The sample oxidized at 500 ℃ for 2 h had good properties of low resistance (0.7 Ωcm), good transmittance rate (above 85%)，predominant excitonic ultraviolet emission with narrow full width at half maximum and weak deep level visible emission. The low resistance p-type ZnO films could be used in short wavelength industrial application.