Aims: The aim of this study is to evaluate the combined application of wet bulb globe temperature (WBGT) and physiological strain Iindices based on heart rate (PSI HR ) for the estimation of heat strain, in hot climatic conditions. Materials and Methods: This cross - sectional study was conducted on 122 men including 71 and 51 workers from the Assaluyeh National Petrochemical Company and Isfahan Steel Company in the center and south of Iran, respectively. The WBGT index, heart rate, and auditory canal temperature were measured at rest and when working. The data were analyzed using descriptive statistics and logistic regression. Results: The results of the logistic regression indicated that the WBGT index was a poor predictor of heat strain and its sensitivity and specificity were 53 and 65%, respectively. However, the combined application of the WBGT and PSI HR indices was a better predictor of heat strain, and the sensitivity and specificity of this combination were 75 and 69%, respectively. Conclusion: According to the results of this study, the combined application of the WBGT and PSI HR indices can be used as a valid estimator of heat strain in hot climatic conditions in the center and south of Iran.