Well-aligned ZnO nanowires have been prepared on sapphire substrate, and structural and optical characterizations indicate that the nanowires are of single crystalline and have relatively high luminescent quality. By employing the ZnO nanowires as an active layer, p-Zn_0.68Mg_0.32O:N/n-ZnO nanowire heterostructure light-emitting devices (LEDs) have been fabricated. The LEDs show pure ultraviolet emission when a forward bias is applied, while the deep-level emission frequently observed in ZnO p-n junctions is almost totally invisible. The devices can work continuously for over 27 h under the injection of a current density of 500 mA/cm², indicating their good stability.