Stress relaxation mechanisms were investigated on three kinds of ethylene–propylene terpolymers (EPDM), differing only in the structure of the crosslinkage, in both nitrogen and air at 160°C. The crosslinking site in these polymers consists of a carbon–carbon bond, a polysulfide linkage, and a carbon–carbon bond and a polysulfide linkage, respectively. Stress relaxation in air for peroxide-cured EPDM was based on the oxidative scission of the main chain. Analysis of the stress relaxation data of sulfur-cured EPDM in nitrogen indicated the presence of stable mono- and disulfide linkages and that the main relaxation process resulted from an interchange reaction between polysulfide linkages. In air, in addition to the interchange reaction, oxidative cleavage of mono and disulfide linkages occurred. Further, the stress relaxation for peroxide–sulfurcured EPDM polymers containing various amount of sulfur was observed in air and nitrogen and was discussed in the connection with highly crosslinked polymers.