Abstract: Cu doped ZnO powder was prepared by the sol-gel method in order to study the origin of room temperature ferromagnetism of ZnO-based diluted magnetic semiconductors. X-ray diffraction spectra show that the solid solubility of Cu in ZnO was 0.08 (mole ratio)；TEM indicate that the Cu/ZnO particles are crystalline and uniform in size. The vibrating sample magnetometer measurements show that the Cu/ZnO powders are of room temperature ferromagnetism. As Cu itself does not have any magnetic properties，the origin of the observed room temperature ferromagnetism of Cu/ZnO could only be attributed to the exchange interaction between the defects of zinc oxide and the doped Cu ions.