Coolant rubber hoses for automobile radiators under thermal and mechanical loadings can be degraded and thus failed due to the influences of contacting stresses of air, coolant liquid and to the locally formed electricity. In this study, degradation behavior of the radiator hose made of EPDM rubber was evaluated. The thermo-oxidative aging test showed that the surface hardness IRHD of the rubber increased together with a reduction of failure strain. By the electro-chemical test it was shown that the penetration of coolant liquid into the skin of the rubber hose arose inducing an increase in weight of specimens as well as a decrease in failure strain and IRHD hardness. The penetration of coolant liquid altered considerably the micro-structure and the micro-hardness distribution along the depth in the rubber hose. On the basis of the above results failure mechanisms of degraded EPDM rubbers were suggested according to the kinds of contacting stresses.