Background: Heat stress is a common occupational health hazard at outdoor workplaces such as mining. The aim of this study was to determine the level of heat stress indices and by measuring physiological Parameters among mine workers and comparing the appropriateness of these indices for measuring heat stress in an iron ore open pit mine. Material and Methods: This cross-sectional study was conducted during summer 2014 on one hundred and twenty male mineworker. Tympanic temperature was measured using a digital thermometer (model FT70) and Heart rate was measured using a digital Emsig device (model BO26). The Physiological strain index was calculated using a standard formula. All environmental and physiological parameters were simultaneously measured and recorded during rest and work. Heat indices were calculated using the related formula. Moreover, the participants completed the Heat Strain Score Index questionnaire. Results: The mean Wet Bulb Globe Temperature index in work state was 30.76 ± 1.56°C, the mean Environmental stress index 29.23 ± 1.26°C, the mean Modified discomfort index 27.75 ± 1.23°C and the mean risk level of Heat Strain Score Index 19.38 ± 4.04. A statistically significant correlation was found between heat stress indices and physiological indices (P<0.01). According to the results of Pearson’s correlation coefficient test, the correlations between Physiological strain index and the indices of Wet Bulb Globe Temperature, Environmental stress index, Modified discomfort index, and Heat Strain Score Index were 0.658, 0.612, 0.614, and 0.417, respectively. Environmental stress index had the highest correlation with the deep body temperature. Moreover, the highest correlation was observed between Wet Bulb Globe Temperature and heart rate and Physiological strain index. Conclusion: Wet Bulb Globe Temperature showed a better correlation with physiological indices. Thus, it can be concluded that Wet Bulb Globe Temperature is a more appropriate index to evaluate heat stress in such workplaces.