Recently, a number of problems have been reported for rubber products in use, even when the products are certified by ozone exposure tests according to ISO 1431-1 and JIS K 6259-1. One of the causes is that the ozone resistance of rubbers can vary markedly depending on the ambient conditions, not only ozone concentration but also temperature and humidity. Therefore, the ozone degradation of rubbers under actual conditions might not be confirmed only by the ozone exposure test at 40 °C under less than the 65%RH recommended by ISO 1431-1 and JIS K 6259-1. The ozone degradation mechanism of vulcanized rubber with or without antiozonants under various temperature and humidity conditions were described.