The current study was conducted to investigate the influence of thermal manipulation (TM) during embryogenesis on hatchability traits, secondary sex ratio and hatchling weights of broiler chicks. A total of 1200 fertile eggs obtained from Cobb-500 breeder broilers parent stock were used in this study. The eggs were randomly distributed into 4 experimental groups. Incubation conditions from day 1 to day 18 were 37.5°C and 52% relative humidity for control group (TM0). while incubation temperature of the thermally treated groups during 13-15 days of incubation was increased to 39ºC for 2 hours in the second group (TM2), 4 hours in the third group (TM4) and 6 hours in the fourth group (TM6). At hatch, the hatchability traits and embryonic mortality (EM) were calculated; all hatched chicks were sorted, sexed and individually weighed. The obtained results showed that, Thermal manipulation did not adversely affect hatchability and embryonic mortality rate, but there was a positive significant effect on the normality of hatched chicks. However there were non-significant differences in the secondary sex ratio of hatched chicks. Thermal treatment did not significantly effect on body weight of hatching chicks.