High quality ZnO thin film was obtained on c-sapphire substrate by using a technique of plasma assisted molecular beam epitaxy (P-MBE), in which MgO and low temperature ZnO are used as buffer layers. High-resolution XRD measurement shows the full width at half maximum (FWHM) of (002) and (102) are only 68.4 and 1 150 arcsec, respectively. In the meantime, atomically smooth surface with root mean square (RMS) surface roughness of 0.842 nm is realized. In addition, Raman and photoluminescence (PL) measurements show that ZnO layer has extremely low stress level and defect density. The realization of high quality ZnO thin film pays a good way for the application of ZnO-based optoelectronic devices.