Silicon nitride (Si3N4), a high-temperature structural ceramic, was synthesized from low-cost rice husk ash (RHA) via carbothermal reduction and nitridation at temperature in the range of 1400°–1470°C for 3–10 h. Three forms of Si3N4 product, i.e., powder mixed with the residual RHA, a layer of single-crystalline whiskers formed on top of the RHA bed, and long polycrystalline fibers formed outside the cavity of the sample holder, were obtained. Both whisker and fiber products could be easily separated from the remaining RHA. Extensive characterizations by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy were conducted to investigate the reaction mechanism. Silicon nitride in both whisker and long fiber forms are suggested to grow via the vapor–solid mechanism, in which silicon monoxide (SiO) vapor is the reaction intermediate. The single-crystalline whiskers are formed directly from the gas phase, but the product in long fiber form grows through the formation of an amorphous intermediate. The condition, which enhances the production of SiO vapor, such as an increased temperature or increased flow rate of the supplied gas, results in significant increase in the yield of both fibrous products, especially in the sample holder placed downstream.