Physico-chemical interactions between hydrophobic polyolefin materials and hydrophilic inorganic nanoparticles such as surface-hydroxylated SiO2 have not been well understood so far. In this study, the effects of particle size and content of SiO2 nanoparticles on isothermal growth rate of PP spherulites in various PP/SiO2 nanocomposites were investigated by polarized optical microscopy. It was unexpected to find that hydrophilic SiO2 nanoparticles can be homogeneously dispersed in a PP matrix. Spherulite growth rates of PP in PP/SiO2 nanocomposites decrease significantly with increasing SiO2 content and decreasing particle size. Most interestingly, the spheralite growth rate was zero for PP/16 nm-SiO2 nanocomposites with SiO2 content above 2.5 wt.-% resulting in a highly transparent film.