Ga-doped ZnO nanostructures with various doping concentrations are prepared by using carbon thermal reduction reaction and in situ doping method. X-ray diffraction measurement reveals only wurzite structures existing in Ga-doped ZnO nanostructures. Scaning electron microscopy observations show that with the increase of Ga doping concentration, the morphology of ZnO nanstrucuture varies gradually from nanorods to nanocones. From the photoluminescence, we find that the visible light emission of ZnO nanostructures can be suppressed obviously and even disappears with the increase of Ga doping concentration. Moreover, the suppression of visible light emission is correlated well with the behavior, and X-ray photoelectron spectroscopy measurement reveals that the vacancy oxygen in ZnO nanostructure decreases drastically with the increase of Ga doping concentration. This result offers a new strong evidence for the mechanism that the visible light emission of ZnO nanostructures is caused by the oxygen vacancy defects. This work also demonstrates that a little Ga incorporation into ZnO nanostructures can effectively reduce the oxygen vacancy defects occurring.