The saturation magnetization of the ZnO nanocrystalline powder prepared by sol-gel method was increased by vacuum annealing, and it can be enhanced again by the further treatment in the oxygen atmosphere at room temperature. The calculations based on density functional theory showed that the O-2 molecule is easy to form the chemical adsorption on the oxygen-deficient surface, and the synergy of the oxygen vacancy and adsorbed O-2 can increase the magnetism.