Producing high-proportion alpha-Si3N4 powder in a fluidized bed is an advanced method that is low cost and high in production. Traditional investigations on silicon powder nitridation isolated the phase transformation from the nitridation process, which brings difficulties in predicting and controlling the produced alpha/beta phase ratio. In this paper, the silicon nitridation process was related to the final alpha/beta phase ratio, and multiple related parameters were discussed. A thermal gravimetric analysis instrument was used to detect the detailed nitridation process. Silicon powders with different specific surface areas (i.e., diameters) were used at temperatures just below their melting point (1410 degrees C). The nonsharp interface model was implemented to simulate the nitridation and predict the ratio of alpha/beta-Si3N4. Some key factors on diffusion and reaction were analyzed. The alpha/beta ratios from simulations were highly consistent with those of X-ray diffraction tests. Mechanisms of powder diameter and thermal diffusion on the production of alpha/beta-Si3N4 were summarized.