Microcellular foamed polypropylene (PP)/SiO 2 composites were prepared by using micro-SiO 2 and nano-SiO 2 particles. The effects of the particle size of SiO 2 on the foaming behavior and mechanical properties of the composites were studied based on heterogeneous nucleation theory. The results showed that the silica facilitated the cell nucleation to some extent. The average cell size of 16.3 μm and the cell density of 4.46 × 10 7 cells/cm 3 were achieved for the composite foam at the silica content of 4 wt.%. The plastic deformation of the PP was strongly constrained due to the presence of the nano-silica. The tensile and impact strength of the nano-SiO 2 composite foam are larger than those of the micro-SiO 2 composite foam due to the high crack propagation resistance in the microcellular PP/nano-SiO 2 composite.