The roles of point defects and defect complexes governing nonradiative processes in ZnO epilayers were studied using time-resolved photoluminescence (PL) and slow positron annihilation measurements. The density or size of Zn vacancies (VZn) decreased and the nonradiative PL lifetime (τnr) increased with higher growth temperature for epilayers grown on a ScAlMgO4 substrate. Accordingly, the steady-state free excitonic PL intensity increased with increase in τnr, at room temperature. The use of a homoepitaxial substrate further decreased the V Zn concentration. However, no perfect relation between τnr and the density or size of VZn, or other positron scattering centers was found. The results indicated that nonradiative recombination processes are governed not solely by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.