Poultry industry is developing in high ambient temperature regions of the world. Many parts of Asia, Africa and South America are located in this climate. Heat stress results in reduced feed intake, body weight gain, carcass yield and increased mortality and FCR and makes it difficult to reach the sufficient poultry meat production in tropical regions (Borges et al., 2004). Vitamin C is one of the most applicable factors to overcome the heat stress in poultry which has been studied in many researches (Brack et al., 1989). The leaves of lemon verbena contain verbena essential oil which consist the components of limonene, citral, geraniol, neral, linalool and researchers has already proven its antimicrobial effect in vitro which would imply stimulating growth performance in vivo (Casamassima et al., 2012). Thaus the aim of this study was to investigate the effect of lemon verbena powder and vitamin C on performance and blood parameters of broilers reared under heat stress. The experiment was carried out with a total of one hundred sixty day-old male Ross 308 broiler chickens in a completely randomized design. They were commercially raised for first 24 days before the beginning of the study. From 25 d of age the birds were assigned to four diets including control diet and treatment diets which supplemented with 2 levels of lemon verbena (0.5 and 1.0%) and one level of vitamin C (250 mg/kg) in 16-floor pens with 10 chickens each and reared to 42 d of age. Temperature initially set at 32 °C for first 3 d of age and decreased by 2.5 °C per week. To induce chronic heat stress, birds were exposed to an ambient temperature of 35±2°C for 8 h daily (from 09:00 AM until 17:00 PM) from 25-42 d of age. Result showed that dietary supplementation of 0.5% lemon verbena increased (P<0.05) average weight gain and feed intake by 5.81 and 3.29%, and reduced feed conversion ratio by 2.59% compared to control group. Casamassima et al. (2012) investigated the influence of 2.5 and 5 mg/kg l
