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Background: In many jobs, individuals are exposed to thermal stress, and thus, evaluation of heat stress is necessary. This study was conducted with the purpose of investigating the relationship of perceptual strain index (PeSI) with other heat indexes under hot laboratory conditions.Methods: This empirical-experimental and cross-sectional study was performed in 5 different temperatures (21 ºC, 24 ºC, 27 ºC, 30 ºC, and 35 ºC). It was carried out on 15 male students in the climate chamber and on the treadmill with the three levels of light (2.4 km/hour), medium (4.8 km/hour), and heavy (6.3 km/hour) activity. The thermal sensation scale and perceived exertion rate were recorded to calculate PeSI and the Heat Strain Score Index (HSSI) was completed. Moreover, wet-bulb globe temperature (WBGT) and air temperature were recorded. Data were analyzed using Pearson correlation test and regression analysis.Findings: Pearson correlation test showed a high correlation (r = 0.84) between PeSI and HSSI (P = 0.001). Furthermore, Pearson correlation test showed, respectively, a good correlation (r = 0.74) and a high correlation (r = 0.81) between PeSI and WBGT, and air temperature (P = 0.001).Conclusion: The results of this study showed that PeSI can be used for the evaluation of heat strain since it has an acceptable correlation with other heat strain indices, can be easily and rapidly applied, and its cost is much less than other heat strain evaluation indices.