BACKGROUND: Ascites is a metabolic disease that causes the economic losses in broiler industry. OBJECTIVES: This study was performed to investigate the effect of threonine and tryptophan on performance, meat quality and quantity, mortality, some enzymes, thyroid hormones, some blood indices and heart parameters of broiler chickens. METHODS: Three hundred one-day-old female chicks (Ross 308) were used in a completely randomized design with four treatments of control (without amino acids) and 140% tryptophan, 130% threonine and combination of both amino acids. Each treatment included the five replicates and 15 birds per each replicate. RESULTS: consumption of tryptophan and threonine caused the improvement of performance, carcass weight and breast meat (P<0.05). The consumption of threonine alone or along with tryptophan decreased the ascites mortality during the whole period (P<0.05). Furthermore, the right ventricle weight to total ventricle ratio was lower in the amino acids (alone or together) fed birds as compared to the birds in control birds (P<0.05). No effects of the amino acids was observed on blood enzymes, thyroid hormones, anus temperature and the blood indices at day 42 of age (P>0.05). CONLUSIONS: taken together, tryptophan and threonine decreases the feed consumption and feed conversion ratio and consequently decrease the ascites mortality in broiler chickens.