Aims: The purpose of this study was to compare performance of two these WBGT measuring devices. Materials and Methods: This study was performed in the climatic chamber located in Health School of Isfahan University of Medical Science. The WBGT Casella and the WBGT Model 686 were placed a wide range of hot/dry (28.5-38.5°C) and hot/wet conditions (25.9-37.7°C) in 3 air flow velocity (0.16, 0.27, 0.31 m/s) for 10 h in the climatic chamber. Wet bulb temperature (t nw ), the dry temperature (t a ), the globe temperature (t g ) and the WBGT were measured every 3 min for both instruments in the total period. The data obtained were analyzed by descriptive method, T-paired test and Regression analysis. Results: The result of this study showed that there were statistically significant differences in the meteorological parameter (t nw , t a , t g , WBGT) obtained from the WBGT Casella and WBGT Model 686 (P < 0.001). A high correlation also was seen between the values obtained by the WBGT Casella and the WBGT Model 686 (r = 0.993). Furthermore, there were statistically significant differences in the air flow velocity parameter (0.157 m/s) and (0.27 m/s) and (0.314 m/s) in a hot/dry (r = 0.994) and (0.980) and (0.994) respectively. There were statistically significant differences in the air flow velocity parameter (0.157 m/s) and (0.27 m/s) and (0.314 m/s) in a hot/wet (r = 0.980) and (0.992) and (0.973) respectively. Conclusion: Since there is a high correlation between the values recorded by two devices and a remarkable cost - efficacy of WBGT Model 686 , using WBGT Model 686 is an acceptable method for measuring the heat stress if the prediction equations are utilized. According to the different temperature conditions, was obtained dry temperature, wet temperature and globe temperature both devices measurement in different climatic conditions equation WBGT Casella = 2.04 + 1.03 (WBGT Model 686 ). About 2 units WBGT Model 686 lower than the WBGT Casella and recommend the measures to be considered in this case as well as the manufacturers also, recommend doing that the reform of sensor measurements and the calculation methods.