High yield orthorhombic single-crystalline KNbO3 nanowires with widths of 60-150 nm and lengths up to a few microns were synthesized with a template-directed process in a hydrothermal reaction. For the first time, Nb2O5 nanobelts were used as reactants as well as templates in the hydrothermal process. The phase of products was determined by XRD, and the morphology and structure were characterized by SEM, TEM, HRTEM, SAED techniques. The growth direction of KNbO3 was determined to be the [001] crystallographic direction. Synthesized nanowires exhibited a second harmonic generation response, an efficient nanoscale second harmonic light source. The excellent nonlinear optical properties of KNbO3 have potential applications in nano-optical devices.