Radiation-induced grafting is a potential method for modification of polymers, for example, highly hydrated polymeric surfaces may be biocompatible and thomboresistant. It is well known that addition of mineral acid to some grafting solution enhanced the radiation grafting yield of some polyolefins. But it turns out not to be the case in all conditions. Some monomers (such as N-VP)can be grafted onto some polymers(such as SPEU)only in basic medium. That means that the acid effect in grafting systems depends on many effects such as component of matrix, type of monomers and solvents, method of irradiation, absorbed dose and so on. In this work, the cuves of grafting yield as a function of absorbed dose showed that there were about two regions of dose: lower than 5 kGy, the grafting yield of AA on silicone rubber was enhanced by acid: the other region, dose growing up to about 13 kGy, the opposite results were obtained. In acid enhancing region of dose the tendency of grafting yield as a function of dose rate and monomer concentration was not influenced by whether the acid existed. The mechanism was discussed as well.