This paper aims to investigate low labor productivity affected by heat stress; focusing on various parameters including outdoor and indoor temperatures and humidity, indoor air quality, and the outfit used. Survey of 130 questionnaires together with measurements of indoor and outdoor air temperatures and humidity were carried out in a garment factory in Phnom Penh, Cambodia during May 2019, which is one of the hottest months of the year. The indoor wet bulb globe temperature, WBGTid, with the evaporative cooling system in operation was ranging from 26 to 34oC; while the outdoor air dry-bulb temperature was around 25 to 41oC. The survey results show that the participants experienced all types of heat-stress symptoms, including heavy sweating, muscle cramps, tiredness/weakness, dizziness, headache, nausea or vomiting, and fainting. Even though the majority of respondents found the workplace temperature comfortable and around 65% could manage bad feelings due to heat stress, about 80% of respondents reported feeling hot and uncomfortable at working place within the period of 1 to 3 months in a year. During the hot months, some 39.6% had feeling thirsty, more than 22% felt exhaustion and had excessive sweating; and around 9% want to move and stay in a comfortable place. To cope with the heat stress, workers mostly drink water, but some also need to move away for a while or remove extra layer of outfit. Finally, noticeable number of workers, 41.3% and 33.7%, respectively, acknowledged heat stress negatively impact their productivity and cause irritation in working together.