The authors report properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy in which two-step growth is employed. They show that the two-step growth is effective in improving structural and optical properties of a-plane ZnO films. Here, the two-step growth is preceded by growing the first layer under Zn-rich (O-rich) conditions and growing the second layer under O-rich (Zn-rich) conditions. All the grown samples show striated anisotropic morphology. The samples with the first, thin, O-rich layer plus the second, thick, Zn-rich layer show smaller root-mean-square (rms) roughness than those with the first, thin, Zn-rich layer plus the second, thick, O-rich layer. The sample with the 20-nm-thick first layer grown under O-rich condition shows the smallest rms roughness of 1.06 nm, which is a smaller rms value than that of the sample grown under the single-step, stoichiometric condition. This sample shows the highest intensity of D (0)X emission at 3.392 eV and small full width at half maxima of (1120) and (1011) x-ray rocking curves, which indicate the good crystal quality.