The effects of various absorbed dose amounts, especially during gamma ray radiation in the presence and absence of oxygen, on the mechanical properties and the dissolution performance of medical rubber products made of chlorobutyl rubber were investigated. From the results of physical property measurements, it was found that the reaction type of chlorobutyl rubber was not a “degradation-type”reaction, but a “crosslinking-type”reaction. Regarding the dissolution performance, the effect of oxygen was the main factor, and when gamma ray was irradiated in the absence of oxygen, it was found that the dissolution performance was almost at the same level as that of original sample. Furthermore, a compound thought to be an oxidant of the triazine vulcanizing agent, which was confirmed by LC-TofMS analysis, was detected only in the case of radiation in the presence of oxygen. This is thought to be due to the decomposition of the cross-linked bonds by the polymer radical reaction accompanying the gamma ray radiation. However, such a phenomenon was not confirmed in the absence of oxygen. These results suggest that the gamma ray sterilization of chlorobutyl rubber in the absence of oxygen enables both sterilization assurance and material degradation prevention. This method can be expected to expand its practical application to medical rubber products in the future.