The ultraviolet light emitting diode based on n-ZnO-nanowire/p-Si heterojunction was fabricated. Quasi-arrays of ZnO nanowires were grown on p-Si substrates using a simple low-temperature hydrothermal method that would be easily extended to mass production. As-grown ZnO nanowires showed good crystallinity, a preferable c axial orientation, and strong ultraviolet emission under optical excitation. Different kinds of cathodes were made to form the electrical contact. The I-V characteristics were diode-like. Under forward bias, the heterojunction diode emitted strong ultraviolet light at 387nm and weaker green light.