This article is concerned with bubble growth dynamics in the CO2/polypropylene microcellular foaming process. The effect of the melt strength on the bubble growth was thoroughly investigated in theory for the first time. The theoretical results indicate that enhanced melt strength effectively restrains the bubble growth and stabilizes the bubble oscillation. Higher melt strength leads to lower bubble growth rate, shorter growth time, and smaller ultimate bubble size. Compared to the melt strength, the viscoelasticity and the gas pressure have less effect on the microcellular foaming process. The bubble growth varies a little as the viscoelasticity is varied. The bubble oscillation and growth rate-are enhanced with increasing gas pressure, which leads to the augmentation of the bubble size. (C) 2010 Wiley Periodicals, Inc. Appl Polym Sci 118: 1949-1955, 2010