Undoped and Ga-doped ZnO (002) films were grown c-sapphire using the pulsed laser deposition (PLD) method. Znvacancy related defects in the films were studied by different positron annihilation spectroscopy (PAS). These included Doppler broadening spectroscopy (DBS) employing a continuous monenergetic positron beam, and positron lifetime spectroscopy using a pulsed monoenergetic positron beam attached to an electron linear accelerator. Two kinds of Znvacancy related defects namely a monovacancy and a divacancy were identified in the films. In as-grown undoped samples grown with relatively low oxygen pressure P(O 2)≤1.3 Pa, monovacancy is the dominant Zn-vacancy related defect. Annealing these samples at 900 o C induced Zn out-diffusion into the substrate and converted the monovacancy to divacancy. For the undoped samples grown with high P(O 2)=5 Pa irrespective of the annealing temperature and the as-grown degenerate Ga-doped sample (n 10 20 cm-3), divacancy is the dominant Zn-vacancy related defect. The clustering of vacancy will be discussed.