Sex determination in chickens has been the subject of investigation. The goal is to comprehend the processes involved in sex determination and the elements that influence how sex is determined. It was recently reported for the first time that temperature affects sex determination in Australian brush turkey (Alectura lathami) (Göth & Booth, 2005). Investigation in quail (Yoshida et al., 1996) and broiler chickens (Collins, 2013; Elmehdawi, 2013) were also reported, but not in Korat chickens. This research aimed to manipulate incubating temperature to determine the male sex in Korat chickens (Gallus domesticus). The effect of increasing/decreasing the temperature from the standard temperature throughout the entire range toward gender determination was studied. The eggs used in this research were Korat chicken eggs. Eggs were incubated at 36ºC, 37.7ºC, and 38ºC (100 eggs per experiment, a total of 300 eggs, randomly assigned). The incubation stages were examined by measuring the degree of the translucence of the eggshell. The incubator temperature was controlled at 36ºC, 37.7ºC, and 38ºC, with the humidity, maintained at 55% throughout incubation. Other factors like ventilation were also regulated to prevent confounding effects. Only eggs developed during the first week (7 days) were used for statistical analysis. Sex was examined twice between days 1 and 21. The first was to investigate the newborn's vent sexing (Tran et al., 2010), and the second by observing sexual dimorphism. Blood was collected from day-old hatched chicks, then extracted DNA using a kit and tested for W chromosome by Polymerase Chain Reaction (PCR) method to confirm sex chromosomes. Testes were randomly collected from 5-week-old chickens that had changed sex. Histology was examined by hematoxylin and eosin staining and then studied by microscopy. The procedures of the experiments were approved in accordance with the advice of the Institutional Animal Care and Use Committee, Suranaree University of Technology, Nakhon Ratchasima, Thailand. The increase in temperature over the period (38.0oC) had a higher percentage of males than females (52.5% versus 47.5%), according to the research findings. P > 0.05 indicates that there was no statistically significant difference. It was discovered that hatched male chicks with high temperatures across the range (38.0oC), as confirmed by the presence of the testicles by histological examination, W chromosomes (female chromosomes), were detected, accounting for 9.7%, when blood from day-old hatched chicks was taken for DNA extraction and W chromosome identification using PCR method. Thus, the incubation temperature, especially the high temperature throughout the period (38.0ºC), can change the sex of the chicks from female to male. It is the first scientifically proven in broilers that incubating temperature manipulation can result in sex change, and the hatching rate is not different from regular hatching. Therefore, controlling the temperature during incubation to determine the sex of Korat chickens to males can generate profits for farmers by setting the temperature at the incubator so that the eggs hatch more males than females. It is also a non-invasive method that does not negatively affect consumers; farmers can do it themselves.