Single-crystalline alpha-Si3N4 nanowires were synthesized by thermal decomposition of a polysilazane preceramic polymer using FeCl2 powders as catalyst. The nanowires, which are 20-40 nm in diameter and up to several mm in length, possess smooth surfaces and uniform diameters along the entire length and contain no detectable structural defects such as dislocation or stacking faults. The study also revealed that all nanowires grow along |100| direction. The growth of the nanowires is attributed to a novel solid-liquid-gas-solid reaction/crystallization process. The mechanism that governs the formation of nanowires rather than nanobelts is discussed.