ZnO thin films were grown on c-plane sapphire substrate using plasma-assisted molecular beam epitaxy. The thickness of MgO buffer layers was optimized for structural and electrical properties of the epi-ZnO films. It is found that with MgO buffer growth time of 60 s, the epi-ZnO film exhibited narrow X-ray diffraction peak as well as low surface roughness. In the meantime, the electron concentration reached a minimum of 2.03 x 10(16)/cm(3) and high mobility of 169.4 cm(2)/V S. These results demonstrate a route to grow good crystals together with excellent mobility and low residual electron concentration, which can ultimately satisfy the requirement for acceptor doping as well as device engineering. (C) 2011 Elsevier B.V. All rights reserved.