Polymer electrolyte membrane fuel cell (PEMFC) is a promising power source. Sealing around the perimeter of the cell is required to prevent the gases/liquids inside the cell from leaking. Polymers are usually used as the seal or gasket materials. The viscoelastic property of polymers can induce stress relaxation of the seal under constant strain. The stress relaxation behavior of ethylene propylene diene monomer (EPDM) rubber, a potential candidate seal in PEMFC, is studied in this paper. The effect of temperature, applied strain, and environmental exposure on Stress relaxation behavior of EPDM is investigated. Using time-temperature superposition, master curves are constructed to estimate the service life of this material used as seals in PEMFC. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.