The ZnO nanowires are treated by the Ar+ plasma and the changes of luminescence properties before and after treatments are analyzed by the test of temperature-dependent spectra under different treatment time. The results show that the near band edge emission intensity at room temperature increases first and then decreases with the increase of treatment time. As for the treatment time of 90 s, the intensity is 2.45 times that of the as-grown sample and simultaneously the defect-related photo-luminescence in visible region is suppressed. The mechanism of plasma treatment is analyzed by the comparison among luminescence spectra at 10 K. When the treatment time is short, the impurities and defects on the ZnO nanowire surfaces can be effectively removed by the Ar+ plasma and thus ultraviolet luminescence intensity is enhanced. When the treatment time is relatively long, the crystal structure is broken due to the introduction of more deep donor-state defects and thus the luminescence property is reduced.