Heat easily degrades a common organic co-agent with a ring structure based on C-N and C-C bonds that is frequently used in conjunction with peroxide-based initiators to cross-link polymer main chains, leading to the shattering of cross-linking points and deterioration of the physical properties of rubbers. In our study, we used co-agents based on phenyl rings that had better bond strengths to improve the heat resistance of co-agents in an effort to maintain the mechanical qualities of rubber at high temperatures. Our results show that the mechanical properties of cross-linked rubbers were considerably maintained after heat treatment at a high temperature. Through extensive investigation, it was discovered that utilizing co-agents with stronger bond strengths caused to reduce the disintegration of the crosslinking sites, which was the basis for retaining mechanical qualities.