Silica gel powders with hydrophilic surfaces were melt-mixed with isotactic polypropylene (PP) using a twin-roller. The microstructure, morphology, dynamic mechanical properties, and tensile properties of the PP/silica composites were thoroughly investigated as a function of the size and content of silica gel particles. Three kinds of silica gel particles with different diameters of 51μm, 100nm, and 26nm were used in this study. It was found that the hydrophilic silica gel could be dispersed homogeneously into the hydrophobic polymer matrix of meso-phase PP. Adding the nano-sized (26nm) silica gel led to a drastic decrease of molecular mobility in the amorphous PP region, resulting in an unexpected significant increase of Young's modulus. The weakening of tensile strength for all the composite samples was demonstrated to be dominantly caused by void-opening in the interface region between the hydrophilic particles and the hydrophobic matrix. This is due to the fact that there are no direct chemical connections between the PP matrix and silica particles.