Abstract: ZnO nanowires were prepared on two piece of Si (100) substrates which faced up and down separately using Au as catalyzer by thermal evaporation and vapor transport. Only (002) diffraction peaks of ZnO can be found on the X-ray diffraction (XRD) patterns, this indicates that ZnO nanowires exhibit (001) preferred orientation. The scanning electronic microscope (SEM) images show that the average diameter is 100 nm and the average length is 4 μm. They are aligned on Si substrate well. While substrates facing up, the ZnO thin film of thickness of 500 nm is deposited on Si substrate firstly and ZnO nanowires grow on the ZnO thin films. And while substrates facing down, epitaxial ZnO nanowires grow on the substrates. The result indicates that the exposure of substrate affects the growth mechanism of ZnO nanowires.