This experiment was conducted to evaluate the effects of supplementary thyme (Thymus vulgaris) essential oil (TEO) and A, C, and E vitamins on performance, egg quality and some biochemical parameters of laying hens under heat stress (HS). A total of 120 White Leghorn laying hens were randomly distributed into 5 experimental groups with 6 replicates and each replicate had 4 laying hens. The HS group was housed at 34 degrees C between 08:00 am and 05:00 pm and the rest of the day at 22 degrees C. The experimental groups were; 1: HS group; 2: HS + TEO (300 mg/kg TEO in diet), 3: HS + TEO + Vit-C (300 mg/kg TEO + 250 mg/kg vitamin C in diet), 4: HS + TEO + Vit-E (300 mg/kg TEO + 250 mg/kg vitamin E in diet), and 5: HS + TEO + Vit-A (300 mg/kg TEO + 15,000 IU/kg vit-A in diet). In the study, body weight, egg weight, feed intake and egg quality of the hens were not affected with treatment groups. Serum glucose level was decreased in the TEO group, plasma MDA was decreased in the TEO and TEO + Vit-C groups and 8-OHdG was decreased in the TEO + Vit-E. It was concluded that it would be more beneficial to use TEO against HS in laying hens with Vit-E and Vit-C instead of using it alone.