Zinc oxide thin films were epitaxially grown at 400 degrees C on C-sapphire substrates by plasma-assisted epitaxy in which elemental zinc is supplied through oxygen plasma excited by radio frequency power at 13.56 MHz. The intensity of bound exciton emission was strongly dependent on the growth rate and was drastically increased with decreasing the growth rate. Surface morphology was also roughened with increasing the growth rate. An initial layer grown with low growth rate around 1.7nm/min at 400 degrees C is quite effective in preventing the unfavorable growth and then to improve the successive growth of a thick ZnO layer with high growth rate. Photoluminescence property and surface morphology were also very much improved with suppressing poly-crystallization in this way in the thick layer grown at high growth rate around 17 nm/min on the initial layer thicker than 300 nm.