Zinc oxide (ZnO) with excellent crystallinity and large electron mobility was grown on aplane (11-20) sapphire (a-Al2O3) substrates by a new chemical vapor deposition method via the reaction between dimethylzinc (DMZn) and high-energy H2O produced by a Pt-catalyzed H2-O2 reaction. The electron mobility at room temperature increased from 30 cm2/Vs to 189 cm2/Vs with increasing film thickness from 0.1 μm to approximately 3 μm. Electron mobility increased significantly with decreasing temperature to approximately 110 – 150 K, but decreased at temperatures less than 100 K for films greater than 500 nm in thickness. On the other hand, the mobility hardly changed with temperature for films lesser than 500 nm in thickness. Based on the dependence of the electrical properties on the film thickness, the ZnO films grown on a-Al2O3 substrates are considered to consist of an interfacial layer with a high defect density (degenerate layer) generated due to a large lattice mismatch between ZnO and Al2O3 substrates and an upper layer with a low defect density.