A dramatic improvement of the toughness of polydicyclopentadiene (PDCPD) composite containing low amounts of vinyl-functionalized SiO2 was achieved, and the toughening mechanism was investigated. Considering the non polarity of the DCPD monomer and the probability of covalent bonding between SiO2 and DCPD, vinyl-functionalized SiO2 (V-SiO2) with a high content of vinyl groups was prepared by a facile, reproducible, one-step, remodeled synthetic sol gel process. The V-SiO2 was characterized with respect to the content of vinyl groups, particle size, and morphology by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), dynamic light scattering (DLS), and field-emission scanning electron microscopy (FE-SEM). PDCPD/V-SiO2 composites were prepared by in situ polymerization. The mechanical properties of the PDCPD/V-SiO2 composites were investigated by universal testing machine (UTM) and dynamic mechanical analysis (DMA). A slight decrease in the yield strength was observed with increasing loading of V-SiO2, whereas the elongation at break increased substantially from 9.0% to 143.4% and the tensile toughness increased by a factor of 14 compared to that of neat PDCPD when just 0.2 wt % V-SiO2 had been added. The dramatic toughness enhancement is attributed to the formation of microvoids and microcracks during the process" of stretching, which can absorb a great deal of energy.