Native defects in ZnO films grown by radio frequency (RF) reactive magnetron sputtering under variable oxygen fraction conditions have been investigated by using monoenergetic positrons beam technique. The results show that the same type defects dominate in these ZnO samples grown at oxygen fraction less than 70% in the proc- ess chamber; and zinc vacancies are preponderant in the ZnO films fabricated in richer oxygen environment. The concentration of zinc vacancies increases with oxygen partial fraction rising. While oxygen fraction reaches 85%

zinc vacancies that could trap positrons decrease

which suggests that impurities could shield zinc vacancies. A com- bination between hydrogen atoms and the dangling bonds in the lattice could weaken the trap of positrons under the 50% oxygen fraction condition. The concentration of zinc vacancies varies in different oxygen fraction films

which is in agreement with the conclusion of photoluminescence spectroscopy.