Synergistic reinforcing and toughening of polydicyclopentadiene (PDCPD) nanocomposites reinforced with multi-walled carbon nanotubes (MWNTs) were achieved and enhancing mechanism was investigated. MWNTs were functionalized with vinyl groups via chemical grafting and the vinyl groups functionalized MWNTs (f-MWNTs) were characterized by fourier transform infrared spectroscopy and field-emission scanning electron microscopy. The mechanical properties of f-MWNTs/PDCPD composites were investigated by means of universal testing, notched izod impact testing and dynamic mechanical analysis. Synergistic improvement of yield strength, tensile toughness and impact strength of composites were observed with the incorporation of 0.1-0.4 wt% f-MWNTs compared with pure PDCPD. The maximum yield strength, toughness and impact strength of the composites increased by about 48%, 94% and 97%, respectively. The simultaneous enhancement is probably ascribed to MWNTs' excellent mechanical properties and high aspect ratio, short and flexible vinyl groups onto MWNTs surface, strong covalent bonds between PDCPD and f-MWNTs.