To obtain polymeric microcellular foam with desirable cell size, the process typically requires higher nucleation rates and higher inert gas concentrations than the conventional processes. A small amount of well-dispersed nanofillers in a polymer matrix were employed to induce heterogeneous nucleation for producing a large number of nucleation sites to facilitate bubble nucleation. The homogeneous nanoparticle dispersion, favorable surface property and particle geometry account for the significant increase in cell density and decrease in cell diameter by adding a small amount of nanoparticles. Moreover, the cell size distribution is another important parameter to characterize the cell morphology in polymer foams. Therefore, shortening of the nucleation time interval is one of the main means to obtain narrow cell distribution in microcellular foams. An increase in cell nucleation rate obviously decreases the time interval for nucleation process, and is expected to narrow the cell size distribution of the final foam. On the other hand, the heterogeneous nucleation induced by the nucleating agents decreases the energy-barrier for cell nucleation dramatically, increases the cell nucleation rate, and is expected to narrow the cell size distribution as well. In this work Microcellular polystyrene/nano-silica nanocomposites were prepared by Pressure dropping process using supercritical CO2 as the blowing agent. Neat Polystyrene foam showed a quite broad distribution of cell sizes. Under the same foaming conditions, the addition of nano-silica resulted in nanocomposite foams having uniform cell size distribution, reduced cell size and increased cell density. The nucleation mechanism was analyzed by the classical nucleation theory. The results indicate that the energy-barrier for heterogeneous nucleation was lower than that of the homogeneous one. The heterogeneous nucleation of nano-silica increased the nucleation rate, decreased the nucleation time interval, and hence facilitated the almost instantaneous growth of cell size. Combined with the well-dispersed nucleation sites, the narrow-distributed cell size was obtained in nanocomposite foams.