Experiments on the synthesis of ultrafine iron particles have been made for the control of particle size distribution using the gas phase reduction of ferrous chloride with hydrogen. The previous studies were focused on the control of particle size of ultrafine particles with the variation of the partial pressure of reactants, residence time of feed, and reaction temperature. However, it is also very important to control the size distribution of ultrafine particles. In this study, the control of particle size distribution was investigated from the standpoint of nuclea- tion. The variation of evaporating condition at the same evaporation rate of ferrous chloride, and of the temperature gradient of reactants between preheating zone and reaction zone were adopted as experimental variables. Ultrafine iron particles having uniform size distribution could be produced under the control of evaporating condition such as the change of the sur- face area at constant evaporating temperature. As the temperature gradient decreased, particle size distribution became uniform and average particle sizes were also decreased.