Polypropylene-grafted nanosilica (PP-g-SiO2) was prepared by us as a new modified nanosilica with long polymer chains and high grafting density. It was found that the addition of PP-g-SiO2 resulted in remarkable strain-hardening behavior of PP. Herein, the foaming behavior of isotactic polypropylene (iPP)/PP-g-SiO2 nanocomposites was investigated by using supercritical carbon dioxide (scCO(2)) as a blowing agent. The results demonstrated that the incorporation of PP-g-SiO2 could obviously enhance the foamability of iPP. In particular, the uniform cell distribution and smaller cell size could be obtained by 1 wt% particle loading, and 5 wt% particle content showed a wider foaming temperature range and higher cell density. The noticeable enhancement in the foamability of iPP was attributed to the reinforced melt strength, high melt elasticity and the increased heterogeneous nucleation caused by well dispersed and long polypropylene chains grafted SiO2. These findings provide new insights to improve the foaming ability of iPP with incorporation of modified nanoparticles.