Abstract. This study aimed to determine the effects of thermal manipulation during early embryogenesis (EE) and late embryogenesis (LE) on hatching weight, body weight at 5 weeks of age, hatchability and embryonic mortality rate in Japanese quails (Coturnix coturnix japonica). Incubation conditions from day 0 to day 17 were; 37.7 °C and 55 % relative humidity for control group. In the thermally treated eggs during early embryogenesis (EE6-EE8 days), incubation temperature was increased to 41 °C and relative humidity to 65 % for 3 hours (12.00-15.00) at 3 consecutive days. Also, in the late embryogenesis stage (LE12-LE14 days), incubation temperature was increased to 41 °C and relative humidity to 65 % 3 hours (12.00–15.00) at 3 consecutive days. At hatch in each trial, all chicks were wing-banded and individually weighted. Thermal manipulations had significant effect on hatching weight, and lowest hatching weights were found in late embryogenesis (LE) group in terms of male and female. In addition, thermal manipulations and gender had significant effect on body weight at 5 weeks of age and lowest body weights at 5 weeks of age were dedected in late embryogenesis (LE) group for both gender.