Heat stress effects on physical and mental health of workers and decreases human function. Asphalt workers are both exposed to the heat of their working process and to the sun heat. This study aimed at evaluating thermal stress and its following function fall and the effect of asphalt work on the degree of heat stress and asphalt workers’ physiological parameters. The present study was done at the work location of 29 asphalt workers in Qum City, central Iran. The degree of thermal stress was by WBGT index. The workers’ physiological parameters such as their blood pressure and pulse rate were studied as well. In order to determine the function fall caused by thermal stress, the related graph was utilized, and the calculated data were analyzed by SPSS software. The longest heat (WBGT) exposure period of asphalt paver drivers, troweling workers, shoveling workers, and burner service workers were 31.59, 32.68, and 33.53 centigrade degrees, respectively causing the laborers’ function fall by 70, 85, and 82%. Increase in WBGT is followed by increase in the average diastolic and systolic blood pressure and pulse rate. There was found a significant relationship between WBGT in the asphalt working locations and out of asphalt working locations (P<0.05). In addition to diseases and effects caused by thermal stress, it is a major reason for man’s function fall and efficiency decline, which eventually causes poor production. Thus, accurate planning and controlling thermal stress are required to prevent occupational diseases and improve human function.