Gas transport properties of N2 and O2 in ethylene propylene terpolymer rubber (EPDM) and high density polyethylene (HDPE)/EPDM blends were determined. The effects of the nature and the degree of crosslinking in both EPDM and HDPE/EPDM blends on gas permeation behavior were examined. The roles of blend composition and morphology on the gas permeation behavior were also investigated. As the volume fraction of EPDM in the blends increased, the permeability increased. Oxygen exhibits a higher permeability than nitrogen because the kinetic diameter of N2 is greater than that of O2. The O2/N2 selectivity of HDPE was higher than that of EPDM and the selectivity decreased with increase in EPDM in the blend. Transport behavior is correlated with the morphology of the blends. Theoretical models proposed by Maxwell and Bruggeman were used to investigate the relationship between morphology and permeation properties.