Using supercritical carbon dioxide as the physical foaming agent, a new batch process was carried out to prepare microcellular polypropylene (PP) and polypropylene/nano-Calcium Carbonate (PP/nano-CaCO3) foams. Four concentrations of nano-CaCO3, 3, 5, 7, and 10 wt% were used. The cell structure of foams and advantages of this new process were investigated and explained by thermal properties. Results showed that the foamed PP/5 wt% nano-CaCO3 produced a microcellular foam with the minimum mean cell diameter (9.55 mu m) and maximum cell density (1.50 x 10(9) cells/cm(3)) among the four blends. Some unfoamed regions were observed in nanocomposite foams because nano-CaCO3 could accelerate crystallization in cooling and cryostat stage. The new process took much less time (2.5 h) to foam and had much broader foaming temperature range (about 55 A degrees C). But the foaming temperature range decreased after blending nano-CaCO3 into PP matrix because nano-CaCO3-induced isothermal and non-isothermal crystallization at higher temperature. In addition, the cell growth effect on variations of volume expansion ratio in PP/nano-CaCO3 nanocomposites could be neglected comparing with the heterogeneous cell nucleation effect.