Single crystalline ZnO nanowires were fabricated on Si (100) substrates by catalystfree high-pressure pulsed laser deposition. It is found that the nanowires start to form when the substrate temperature and growth pressure exceed the critical values of 700 o C and 700 Pa, and their size strongly depends on these growth conditions. That is, the aspect ratio of the nanowires decreases with increasing temperature or decreasing pressure. Such a size dependence on growth conditions was discussed in terms of surface migration and scattering of ablated atoms. Room-temperature photoluminescence spectrum of ZnO nanowires shows a dominant near-band-edge emission peak at 3.28 eV and a visible emission band centered at 2.39 eV. Temperature-dependent photoluminescence studies reveal that the former consists of the acceptor-bound exciton and free exciton emissions; while the latter varies in intensity with the aspect ratio of the nanowires and is attributed to the surface-mediated deep level emission.