ZnO thin films are prepared on alpha-Al2O3 (0001) substrates by zinc evaporation in O2N2 mixed radio-frequency plasmas. Films can be deposited at N-2 mixing ratios within 50% in the case that the substrate temperature is 400 degrees C. The ZnO films are c-axis oriented, and all the transmission spectra agree with the typical spectrum of ZnO. The electrical properties change to a large degree by annealing in air. After the annealing, carrier densities of the films deposited at N-2 mixing ratios under 25% tend to decrease as increasing the N-2 mixing ratio, and they increase abruptly again when the N-2 mixing ratios are 25% and over. Photoluminescence spectra of the films are also measured, and nitrogen-mixing effects on their properties are investigated.