The objectives of this research were to study the correlations of heat strain score index (HSSI) and values of wet-bulb globe temperature (WBGT) index with physiological parameters such as core body temperature, heart rate, and blood pressure of workers exposed to heat stress in a glass manufacturing plant in order to determine the reliability of HSSI. This cross sectional study was carried out on 72 production, packing, and quality control male workers of a glass manufacturing plant. The WBGT values were recorded according to standard No. International Standard Organization (ISO) 7243 heat stress standard during the summer season in July–August 2013. Metabolic rate for each task was estimated according to standard No. ISO 8996. Body core temperature and heart rate were monitored according to standard No. ISO 9886. HSSI questionnaire was used to evaluate heat strain. The data were analyzed using Pearson and Spearman correlation coefficients and linear regression test. The maximum WBGT value (30.20±1.06ºC) was recorded in production unit of plant. The scores of HSSI index in production, packing, and quality control operators were 23.16±1.9, 21.56±1.69, and 21.43±2.04, respectively. The highest correlation coefficient was found between the measured WBGT values and core body temperature (r=0.462). Positive correlations between the values of WBGT index and studied physiological parameters indicated that the WBGT as the most widely accepted index is better than HSSI for the assessment of thermal stress in glass manufacturing industry.