This experiment was conducted to investigate the effects of leucine (Leu) supplementation on performance and carcass traits of broilers along with the re-evaluation of standardized ileal digestible (SID) Leu requirement of broiler chicks on days 0-14. Birds were fed seven experimental diets with SID Leu levels of 16.3 (basal), 17.3, 18.3, 19.3, 20.3, 21.3, and 22.3 g/kg. Some 280 male broiler chicks were randomly assigned to the seven dietary treatments in a completely randomized design with five replicates. Dietary SID Leu levels had significant effects on body weight gain(BWG), feed conversion ratio(FCR), carcass, and breast percentages. BWG increased (p<0.05) when the broilers fed diets with 18.3 to 21.3 g/kg SID Leu compared to the basal diet. FCR decreased (p<0.05) by dietary SID Leu levels of 18.3 to 21.3 g/kg in comparison with the basal diet. Carcass yield increased (p<0.05) by SID Leu levels of 19.3 to 21.3 g/kg. Breast percentage increased (p<0.01) when the broilers received diets with 18.3 to 22.3 g/kg SID Leu and the highest value obtained by the SID Leu level of 21.3 g/kg. Abdominal fat decreased linearly by Leu supplementation (p<0.01). Regression analysis indicated the SID Leu requirement of 18.67, 18.88, and 19.85 g/kg for BWG, FCR, and breast meat yield, respectively. It can be concluded that broiler performance characteristics, especially breast yield, improve by Leu supplementation. Based on the results, the Leu requirement of modern broilers has increased dramatically due to genetic selection for a higher growth rate.