This research was carried out to determine the effects of cinnamon oil (Cinnamomum zeylanicum L.) added to diet at various concentrations on antioxidant activity, serum glucose, triglyceride, HDL, LDL, and total cholesterol levels in heat-stressed quail (Coturnix coturnix Japonica). Total of 180 quails, fifteen-days-old, were allocated into 6 groups in 2x3 factorial order. Each group was consisted of 10 birds for each of the 3 replicates. The groups were balanced according to gender and inital live weight. Basal diet was given to the control groups in both thermo-neutral (TN) and heat-stress (HS). The birds were fed with the basal diet supplemented with 250 or 500 ppm cinnamon oil in the other experimental groups. The cinnamon oil used at each concentration decreased serum glucose, triglyceride and LDL cholesterol levels in the groups exposed to the heat stress (P<0.01) while the heat stress increased serum glucose (P<0.001), triglyceride (P<0.01) and LDL cholesterol levels (P<0.001). Total cholesterol level (P<0.01) increased with the effect of heat stress, while HDL cholesterol level decreased (P<0.01). The serum malondialdehyde level (P<0.05) increased with the effect of the heat stress. While the serum glutathione peroxidase (GSH-Px) activity (P<0.001) decreased with the effect of the heat stress, the feed additive increased the serum glutathione peroxidase activity (P<0.001), depending on the concentrations used. The serum glutathione and superoxide dismutase activities (P<0.01) decreased with the effect of the heat stress while increase in both parameters in the group of TN500 was found (P<0.01). As a result, it was concluded that the addition of cinnamon oil to the diet may reduce the negative effects of heat stress which is an important stress factor in quails.