ZnO epilayers with a thickness of ∼360 nm were directly grown on r-sapphire planes at different temperatures (550, 500, 450 °C) with metal-organic chemical vapor deposition. Residual strains along the vertical direction and optical properties of the epilayers were investigated by using a variety of techniques including x-ray diffraction, low-temperature photoluminescence (PL), cathodoluminescence and Raman scattering. Compressive strains were revealed to exist in all the epilayers. Their strengths were found to reduce with increasing growth temperature. In addition, the optical properties of the epilayers were studied and the relationship between the strain and PL peak position was unveiled. © 2012 IOP Publishing Ltd.