In the carbothermal reduction-nitridation method of producing silicon nitride powder, the effects of addition of silicon nitride seeds on the particle size and α-content in the resulting silicon nitride powder were investigated. The ratio of carbon to silica was determined to be 2.2, which is somewhat higher than the stoichiometric ratio. The ratio was fixed in the subsequent experiments. Adding a small amount of seeds caused a steep increase of α-type in the resulting silicon nitride powder regardless of the types of seeds. However, as the amount of seeds increased, the powder began to reflect the original crystal form. The seed prepared through a pulverizing process yielded fine silicon nitride particles of 0.5 to 0.6 μm, equal to the mean diameter of the seed. It is inferred that this was due to the existence of a large numbers of very fine particles in the seed powder. The seed made by the silicon-imide decomposition method produced relatively large particles because of the lack of very fine particles, the size being twice of the mean diameter of the seed. The relation between the numbers of seeds and resultant particle size (D) could be described by the equation D=k(1/N) 1/3 , where Nis the numbers of seeds and k is a constant.