Abundant Zn vacancies (7.5 mol%) were successfully introduced into undoped ZnO by a simple solvothermal method followed with thermal calcination, and undoubtedly proved by both characterizations and computations. The presence of Zn vacancies led to some new properties in ZnO, such as p-type conductivity, room-temperature ferromagnetism and high photocatalytic activity. The formation process of abundant Zn vacancies during the synthesis of ZnO was also discussed. This work demonstrates that metal defects can be easily engineered in undoped metal oxides, which may trigger many unexpected behaviors and thus widen the synthetic approach and application of functional materials in energy fields.