The simultaneous effect of ozone concentration and temperature in an ozone test of rubber sheets for roof membranes were estimated in this study. The ozone tests with five levels of ozone concentration from 10 pphm to 200 pphm were first carried out at 40℃ to know the effect of ozone concentration. The times to cracking or rupturing became shorter according to increase of ozone concentration. This relation can be expressed by the following equation. t = 10^B/X^A, where t is time to cracking or rupturing (hrs), X is ozone concentration (pphm), A and B are material constants depending upon kinds of rubber sheets. Next, the ozone tests at three ambient temperatures such as 40, 60 and 80℃ were performed to estimate the effect of temperature to ozone degradation. It was made clear that the same effect as ozone concentration was observed in these tests and a rise in temperature accelerated ozone degradation. And an following equations including both parameters, ozone concentration and temperature was formulated from these tests results obtained. t = 10^<(C/T+D)>/X^A, where T is absolute temperature (K), X is ozone concentration (pphm), and A, C and D are material constants. Finally, the environmental condition in the ozone test prescribed in Japanese Industrial Standards for rubber sheets was estimated by comparison between the degradations calculated by the test condition and outdoor condition.