Based on first principle FP-LAPW calculations, the electronic structure and magnetic properties of ZnO with Zn cation vacancy has been investigated. We find that the cation vacancy defect can induce a magnetic moment of about 2μB/supercell. The magnetic moment mainly comes from 2p-orbitals of O atoms which surround the Zn vacancy. We also find that the two Zn vacancies in ZnO always coupled Ferro-magnetically. Furthermore, the system shows half metallic Ferro-magnetic properties with high Curie temperature.