The profile of ozone formation in a propene-NO(x)-air-irradiation system was studied at 20, 30, 40, and 50-degrees-C in order to clarify the effect of temperature. The higher the temperature, the faster the formation of ozone and the longer the duration of a high concentration of ozone. The former phenomenon was due to the effect of temperature on the stability of HOONO2, and the latter was due to faster decomposition of PAN at higher temperatures. Computer simulation was employed to realize the mechanism for those temperature effects.