Heat stress particularly at outdoor workplaces in hot-dry climates can disrupt the physical or mental functions and effect negatively on human performance. We aimed to study the effect of heat stress on productivity loss among workers of brick workshops in Qom Province – central Iran. This cross-sectional descriptive and analytic study was performed on 184 workers in 40 brick workshops. Environmental parameters including dry temperature, natural wet-bulb temperature, globe temperature, relative humidity and air velocity were measured at three times during work shift. WBGT (Wet Bulb Globe Temperature) heat stress index was calculated based on equation provided by Australian Bureau of Meteorology. Potential labor productivity losses due to heat stress were determined using a graph presented already. The total average of WBGT was 31.84±3.46°C. WBGT index according to different jobs was 39.48±8.2°C, 29.86±2.79°C, 29.31±0.97°C and 28.69±1.89°C for working in kiln, material handling, working on conveyor and making adobe, respectively. The average loss productivity in mentioned tasks was 93.57±16.54, 66.58±20.58, 65.53±15.26 and 48.27±13.04 respectively, with total average of productivity loss 68.48±16.35%. The statistically significant relationships were found between WBGT values and percentage of loss productivity in total and for all understudy jobs (p<0.001). Heat stress can lead to decrease in labor productivity at outdoor workplaces like brick industries. According to climate change in the future and gradual global warming, paying more attention to the occupational health issues in outdoor workplaces at dry and warm environment seems to be more important than before.