A model was proposed to predict the particle size distribution in a steady state continuous CVD fluidized bed reactor and in outflow stream by variation of feed rate of input stream. In CVD fluidized bed reactor high purity polycrystalline silicon particles were grown by a pyrolysis of silane gas （SiH₄） and chemical vapor deposition (CVD) on seed particles. It was found from the computer simulation that the average size and output rate of product particles increased with decreasing silicon feed rate, and that the particle size distribution was broadened by the decrease of the feed rate, which is unfavorably influenced in a continuous CVD fluidized bed reactor. It was also demonstrated that the simulation results were in good agreement with experimental ones on a 4 inch CVD fluidized bed reactor and the model could be used in the estimation of operation conditions of CVD fluidized bed reactor.