Microcellular injection molding of neat isotactic polypropylene(iPP) and isotactic polypropylene/nano-calcium carbonate composites(i PP/nano-CaCO_3) was performed using supercritical carbon dioxide as the physical blowing agent. The influences of filler content and operating conditions on microstructure morphology of i PP and i PP/nano-CaCO_3 microcellular samples were studied systematically. The results showed the bubble size of the microcellular samples could be effectively decreased while the cell density increased for i PP/nano-CaCO_3 composites, especially at high CO_2 concentration and back pressure, low mold temperature and injection speed, and high filler content. Then Moldex 3D was applied to simulate the microcellular injection molding process, with the application of the measured ScCO_2 solubility and diffusion data for i PP and i PP/nano-Ca CO_3 composites respectively. For neat i PP, the simulated bubble size and density distribution in the center section of tensile bars showed a good agreement with the experimental values. However, for i PP/nano-CaCO_3 composites, the correction factor for nucleation activation energy F and the pre-exponential factor of nucleation rate f_0 were obtained by nonlinear regression on the experimental bubble size and density distribution. The parameters F and f_0 can be used to predict the microcellular injection molding process for i PP/nano-CaCO_3 composites by Moldex 3D. Microcellular injection molding of neat isotactic polypropylene (iPP) and isotactic polypropylene / nano-calcium carbonate composites (i PP / nano-CaCO 3) was performed using supercritical carbon dioxide as the physical blowing agent. The influences of filler content and operating conditions on microstructure morphology of i PP and i PP / nano-CaCO 3 microcellular samples were studied systematically. The results showed the bubble size of the microcellular samples could be decreased rather the cell density increased for i PP / nano-CaCO 3 composites, especially at high CO 2 concentration and back pressure, low mold temperature and injection speed, and high filler content. Then Moldex 3D was applied to simulate the microcellular injection molding process, with the application of the measured ScCO 2 solubility and diffusion data for i PP and i PP / nano-Ca CO 3 composites respectively. For neat i PP, the simulated bubble size and density distribution in the center section of tensile bars s However, for i PP / nano-CaCO 3 composites, the correction factor for nucleation activation energy F and the pre-exponential factor of nucleation rate f_0 were obtained by nonlinear regression on the experimental bubble size and density distribution. The parameters F and f_0 can be used to predict the microcellular injection molding process for i PP / nano-CaCO_3 composites by Moldex 3D.