Polyurethane dispersion (PUD) was prepared with polycarbonate diol (PCD), isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA) in a 8 L pilot reactor. The end point of prepolymerization step was measured by the change in the torque of the stirring motor which is related to the viscosity of reaction mixtures. The changes in the viscosity of reaction mixtures, particle size distributionof PUD, water evaporation rate in film casting and thermal properties of cast film were investigated as varying the stirring rate in dispersion step. During the dispersion step, the viscosity of reaction mixture increased initially, and then decreased sometime later. The extent of viscosity increase and particle size of PUD decreased with the stirring rate. As the particle size became smaller, the evaporation rate decreased. But thermal properties of cast film such as thermal decomposition temperature and glass temperature were not affected by the particle size of PUD.