With Lewis Acids as catalysts in melt system, the influence of kinds of Lewis Acids, dosages of catalysts on the behaviors of crosslinking and grafting of ethylene-propylene-diene rubber (EPDM) were investigated. The Lewis Acids, such as anhydrous AlCl3, FeCl3, SnCl4, could initiate the crosslinking of EPDM and the grafting between EPDM and polystyrene (PS). The carbon-carbon double bonds existing on EPDM chain were favorable to the formation of the initial carbocation in the presence of Lewis Acids. The carbocation initiated carbonium ion polymerization between the unsaturated bonds, or substituted for a proton from the phenyl in the presence of PS forming EPDM-g-PS copolymer. Anhydrous aluminum chloride was found to be an efficient catalyst and its initiating temperatures for crosslinking or grafting were about 110 degrees C. The amounts of gel and the data of torques showed that there was a competition between the crosslinking-grafting reaction and the degradation of blending components in the presence of AlCl3. The EPDM-g-PS copolymer served as a compatibilizer in the EPDM/PS blends and enhanced the mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2961-2967, 2011