α-silicon nitride single-crystalline nanowires were prepared by direct nitridation of granulation Si powders in N₂-H₂ mixture gas. The nitridation product has the core-shell structure (Si₃N₄ nanowires @porous Si₃N₄ powders), where Si₃N₄ nanowires can be effectively separated by crushing and grinding. The results show that the as-prepared α-Si₃N₄ nanowires are straight and uniform with diameters of 80-150 nm, length to diameter ratios of 20-50, purity higher than 95wt%, and yield of 3.1%. Further study indicates that growth of Si₃N₄ nanowires is controlled by the Vapor-Liquid-Solid (VLS) mechanism, where the trace Fe elements serves as catalyst in the reduction atmosphere. In this study, The process of the raw silicom powder granulation after nitriding shows three advantages: 1) remarkably increasing growth space of nanowires; 2) leading to concentrated distribution of nanowire, tacilitating subsequent separation; 3) remarkably increasing the nitridation rate.