Microcellular polycarbonate/nano-silica nanocomposites (PCSN) were prepared by temperature rising process using supercritical CO2 as the blowing agent. Neat PC foam showed a quite broad distribution of cell sizes. Under the same foaming conditions, the addition of nano-silica resulted in PCSN foams having uniform cell size distribution, reduced cell size of 0.3-0.5 mu m and increased cell density of 10(11)-10(13) cells/cm(3). The underlying nucleation mechanism was semi-quantitatively analyzed by the classical nucleation theory. The results indicate that the energy-barrier for heterogeneous nucleation was three orders of magnitude lower than that of homogeneous one. The heterogeneous nucleation of nano-silica aggregates dramatically 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, resulted from the uniform dispersion of nano-silica aggregates, the narrow-distributed cell size was obtained in PCSN foams. (c) 2006 Elsevier Ltd. All rights reserved.