BACKGROUND: Searching for factors that reduce the adverse effect of heat stress (HS) on broiler performance has particular importance. ObjectiveS: The present study was designed to investigate the influence of dietary melatonin, gamma-amino butyric acid (GABA) and dark period on growth performance, carcass characteristics and blood parameters of broiler under heat stress condition. Methods: A total of 143 one-day-old chickens (broiler line Arian) were used in an unbalanced completely randomized form with 4 treatment, 13 experimental units and 11 birds per experimental unit. The treatment consists of 40 mg of melatonin/kg diet from the age of 30 to 40 days, 100 mg (GABA)/kg diet from the age of 10 to 40 days, dark of period from 10 to 40 days of age and control. All treatments were kept a circular HS (40±1°C) for 6 h from 35 to 40 post hatch. Weekly growth performance of chickens was determined and blood samples were taken, on days 10, 30 and 40 of age for measurement of glucose, cholesterol, triglyceride, HDL and LDL parameters. On days 35 and 38 of age T3, Thiobar Butyric Acid Reacting Substances (TBARS) and corticosterone parameters were measured. Data were analyzed by using of GLM procedure of SAS. ResultS: The diet supplementation of GABA increased feed intake in the second (p<0.01) and third (p<0.05) weeks of age. Dark period reduced body weight in the third weeks of age (p<0.05). The treatment of GABA in the fourth week increased body weight compared to the control group (p<0.01) and a significant decrease for FCR was observed in the fourth weeks (p<0.01). Addition of GABA in diet increased serum glucose at 30 days of age (p<0.01), in the both melatonin and GABA treatment groups serum HDL was increased at 40-day-old (p<0.01). Control chickens showed significantly (p<0.01) higher TBARS compared to the experimental group birds at 38-day-old. Conclusions: The results demonstrated that dietary GABA had the most beneficial effects in reducing the adverse effects of heat stress on the broiler chicken.