Aims: The purpose of the present study was to evaluate the effect of fabric stuff of work clothing that are widely used in Iran industries on the physiological strain index (PSI) at hot conditions in the climatic chamber. Materials and Methods: This interventional study was performed upon 18 male students in 16 trials, which included combination of four kinds of work clothing (13.7% viscose (VIS) 86.3% polyester(PES), 30.2% cotton [CT]-69.8% PES, 68.5% CT-31.5% PES, 100% CT, two activity levels (light and moderate) and two kinds of climatic conditions included hot-wet (T a = 35, RH = 70%) and hot-dry (T a = 38, RH = 40%). During each trial, the RH and core temperature was recorded once a minute and then PSI was calculated. Data were analyzed by using SPSS-16 software. Results: The results showed that in hot-wet conditions, the least value of PSI in light and moderate activities was related to 100% CT clothing and 30.2% CT-69.8% PES clothing, respectively. In hot-dry conditions, the least value of PSI in both of activities was related to 30.2% CT-69.8% PES clothing. The mean value of PSI in hot-wet conditions, during moderate activity had significant difference for various clothing types (P = 0.044). Conclusion: The research findings showed that for a heat strain reduction in hot-wet conditions at light activity level, 100% CT clothing is suitable. Furthermore, at moderate activity level, 30.2% CT-69.8% PES clothing and in hot-dry conditions, 30.2% CT-69.8% PES is suitable.