The objective of this paper is measuring and working out calculation model of chlorine dioxide (ClO2) gas reaction rate constant with the bag method. The experiment was conducted using a Tedlar (polyvinyl fluoride) bag to measure the ClO2 concentration decay in detail and to calculate the reaction rate constant. The Tedlar bag into which the ClO2 gas mixed with the clean air was introduced was set in a thermostatic chamber. The concentration of the ClO2 gas was measured every one hour for six hours. Air temperature, relative humidity, absolute humidity and light irradiation were set as experimental variables. High air temperature and light irradiation accelerated the decrease of ClO2 gas concentration; the latter had the greatest influence on the decrease of ClO2 gas concentration. A mathematical expression as a function of air temperature and light irradiation contribute to ClO2 gas absorption was proposed to predict reaction rate constant of ClO2 gas.