ZnO thin film that grows at low temperature benefits the sharp interface in heterostructure and enough doping level of acceptor in ZnO. In general, the smooth growth can be easily obtained at temperature higher than 600°C, but difficult in the case of low growth temperature. In this work, by controlling the growth ambient and growth rate, a series of ZnO thin films with smooth surface were grown on a-plane sapphire substrates at 450°C by the plasma-assisted molecular beam epitaxy (P-MBE). The growth was performed in oxygen-rich atmosphere. To tune the growth rate, the zinc flux was changed by varying the K-cell temperature of zinc source (T) while keeping oxygen flux constant. The growth rate of the samples is only 40~100 nm/h. Scanning electron microscopy (SEM) images indicate that there are lots of irregular grains on the thin film surface at high zinc flux, and most of the grains disappear gradually from the surface with the zinc flux decreasing. This smooth growth with low growth rate is conducive to control finely the layer thickness and smoothness in the multilayer structure. The root mean square (RMS) surface roughness is only 0.238 nm, measured by atomic force microscopy (AFM). The smooth surface benefits from the low growth rate and small sub-lattice mismatch between the c-plane-ZnO thin film and a-plane-sapphire substrate.