Processing of microcellu1ar foam was studied far polyurethane. In order to increase the nucleation rate in a gas supersaturated resin, ultrasonic excitation was applied to the mixture of polyol and isocyanate. Microcellular structure was produced by supersaturation of the polyol resin with nitrogen gas at elevated pressure and ultrasonic bubble nucleation right after the impingement mixing of two components of the polyurethane system. Ultrasonic excitation was applied to the resin saturated by nitrogen at low saturation pressure. Assuming the bubble growth was controlled by diffusion, theoretical prediction was carried out numerically to understand the bubble growth mechanism in the cavity during mold filling. Final bubble sizes were calculated by considering the gelation time and the diffusion boundary. Variation in the viscosity of polyurethane during polymerization reaction was predicted by considering reaction kinetics and the gelation time was determined. The diffusion boundary was predicted based on the number of nucleated bubbles which were determined both theoretically and experimentally.