Abstract: A mathematical model describing the coagulation and growth of ultrafine particles was derived. The effects of the operation parameters on the ultrafine particle size and its distribution were investigated. The results showed that increasing of the concentration of the reactants enhanced the coagulation growth rate which induced large particle size and wide distribution. The growth rate was insensitive to reaction temperature so that the temperature had less effect on particle properties at high super-saturation level. The particle size became larger and its distribution became wider and then became self-preserving distribution with extending the reaction time. The principles of CSD control of ultrafine particles were proposed according to the model.