Abstract Undoped ZnO films grown on sapphire by pulsed laser deposition are magnetic at room temperature. A comprehensive study involving x-ray diffraction, positron annihilation spectroscopy, and superconducting quantum Interference device-vibrating sample magnetometer is performed to study the origin of the observed magnetization. Correlations between the saturation magnetization, V_Zn−2V_O concentration and surface to volume ratio of the grain found experimentally show that the magnetization is associated with the vacancy cluster and probably V_Zn−2V_O residing on the grain surface.