Silicon nitride nanowires have been synthesized by mechanosynthesis method. The silicon powders were first milled for 160 hours under the presence of ammonia gas with 300kPa. And then they were heat-treated at 1250 for 2 hours in the pure nitrogen gas. The phase and the microstructure of the as-prepared product were detected by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results showed that the product consisted of α-Si3N4 nanowires. The nanowires had the length of tens of millimeters and the diameters of 50～100nm. The vapor-solid (VS) mechanism was employed to interpret the nanowires growth.