Presented is a model for the prediction of the particle size distribution of inorganic powders made by the gasphase reaction. The assumption used in previous models that a mo- nomer forms a nucleus is eliminated; the model incorporates the determination of the number of monomers required to form a nucleus and the nucleation rate based on the classical theory. The present model was applied to a manufacture of ultrafine iron powders by hydrogen re- duction of FeCl₂ vapor. The average particle size of the iron powder predicted by the model was one half to two thirds of the experimental value. For a reaction temperature of 900℃, the model could fit the experimental diameter by using the surface tension of iron 1,283 times as high as that in the literature. The model prediction agrees well with experimental obser- vation in the trend of particle size increasing with the feed rate of FeCl₂. The model, how- ever, showed a deviation from experimental observations on the effects of the reaction tem- perature and of the preheating temperature on the particle size. Further investigation needs to be followed to correct the model.