In this report, large-scale vertically aligned ZnO nanowires, with diameter around 75 nm and length around 2-5 μm, were synthesized on a-plane sapphire by a single step chemical vapor deposition method. The XRD pattern of the as-prepared sample showed a strong ZnO (0002) peak and a weak ZnO (0004) peak that indicate good orientation and high crystal quality of the ZnO nanowires. The sample was then treated by hydrogen plasma, without exhibiting obvious structural damage to the nanowires. The photoluminescence spectra of as-prepared and H2-plasma-treated samples were then examined. A strong green emission peak (centered at 520 nm) was observed in the PL spectrum of as-prepared sample. In sharp contrast, a significant increase of the near-band edge emission (centered at 380 nm) and a strong decrease of the green emission (centered at 520 nm) were found in the PL spectrum of H2-plasma-treated sample. We propose that an efficient passivation of oxygen vacancies by H atoms will cause a drastic decrease of the green emission. More important, it would lead to a significant reduction of surface depletion layer, leading to a great enlargement of total effect area for UV emission. Meanwhile, the significant enhancement of the intensity of UV emission might also attribute to the combined effects of structure-induced waveguide behavior and UV amplified spontaneous emission. It is expected that the enhanced UV emission of vertically aligned ZnO nanowires can be used to improve the performance of UV light emitting devices.