Ozone degradation of vulcanized EPDM (ethylene-propylene-diene rubber) was investigated by changing atmospheric humidity. Ozone exposure test of a carbon black filled EPDM vulcanizate was carried out with 100 pphm ozone at 40 °C, for 168 hours under relative humidity (RH) at 80 and 20%, after a strain of 20% stretching. The degradation of the vulcanizates was assessed by morphology observation with digital microscopy and SEM (scanning electron microscopy). Ozone cracks did not appear on the surface of the vulcanizates exposed with ozone at 80%RH and 20%RH, but white blooms identified as ZDMC (zinc dimethyldithiocarbamate) appeared on the surface of them. The ozone degradation of the EPDM was attributed to a chain scission of the rubber molecules by hydroxyl radical generated due to a reaction of moisture and ozone.