ZnO single crystals were implanted with He ions of energy of 20—100keV. The total implantation dose was 4.4×1015cm-2. Doppler broadening of positron annihilation spectra were measured using a slow positron beam to study the implantation-induced defects. The results suggest that after implantation, divacancies or larger vacancy clusters are produced. After annealing below 400℃, He impurity begins to occupy the vacancy clusters. Upon further annealing above 400℃, the vacancy clusters grow in size. At annealing temperature of above 800℃, He atom is released from the vacancy clusters, and the vacancies begin to recover and are annealed out at 1000℃.