Filler functionlizetion is an important strategy to enhance the mechanical properties of polymer composites via the realization of polymer/filler coexists compatibly, interfacial bonding and efficacious load transfer between matrix and filler. In this study, to realize the tailored mechanical properties of polydicyclopentadiene (PDCPD) and to gain insight into the effect of functional groups on their properties, functionalized silica (f‐SiO2) with different functional groups is prepared and combined with PDCPD to produce composites. Compared with neat PDCPD, the yield strength of the composite with 0.20 wt% vinyl‐SiO2 decreases and the impact toughness enhances limitedly, whereas the tensile ductility improves by 16 times. For the case with 0.20 wt% phenyl‐SiO2, it is interestingly observed that the yield strength reinforces by 45.3% and the impact toughness increases remarkably by 222.8%. For the case containing 0.20 wt% ethyl‐SiO2, simultaneous promotion in terms of strength and toughness is achieved. More importantly, the reinforcing/toughening and reaction mechanisms of f‐SiO2/PDCPD composites are explored.