EPDM-graft-methyl methacrylate and styrene (EPDM-g-MMA-St) was synthesized by solution graft copolymerization of methyl methacrylate (MMA) and styrene (St) onto ethylene-proplene-diene terpolymer (EPDM) in toluene /n-heptane cosolvent using benzoyl peroxide as an initiator. Fourier transform infrared spectroscopy provides a substantial evidence of grafting of MMA and St onto EPDM. EPDM-g-MMA-St/MS resin blends (MES) were prepared by melt blending EPDM-gMMA-St and MS resin, and the toughening effects of EPDM-g-MMA-St on MS resin were studied. The results showed that the synthesized conditions of EPDM-g-MMASt influenced the toughening effect of EPDM-g-MMA-St on MS resin. Notched Izod impact strength of MES increased with increasing grafting ratio, grafting chain polarity of EPDM-g-MMA-St, and EPDM content in MES. Differential scanning calorimetry showed that EPDM-g-MMA-St and MS resin are compatible partially and the compatibility improves with increasing grafting chain polarity of EPDM-g-MMA-St. Transmission electron microscopy and scanning electron microscopy analysis showed that the phase structure was "sea-island" structure, and the particle diameter of EPDM-g-MMA-St increased, meanwhile, surface to surface interparticle distance decreased with an increase in EPDM content, which resulted in the toughening mechanism of MES changed into slight shear yielding of matrix from the damage mode of cavitation. (c) 2008 Wiley Periodicals, Inc.