Two experiments were conducted to determine lysine (Lys) requirements of two commercial strains, Ross 308 and Cobb 500, from 15 to 28d of age. Basal diet was formulated to be adequate in all nutrients except for Lys. Incremental levels of supplemental Lys were added to the basal diet generating six experimental treatments in a range from 0.60 to 1.20% digestible Lys. Birds were randomized across 48 floor pens (4 replicates and 12 birds in each replicate) in completely randomized design and each pen was fed one of six amino acid levels. Body weight gain, feed intake, feed conversion ratio, and digestible Lys intake responded quadratically to graded levels of digestible Lys, and interaction effects between Lys and strain were significant for all performance traits except for feed conversion ratio. Digestible Lys requirements were estimated using broken-line linear and broken-line quadratic models. In Ross 308, digestible Lys requirements for body weight gain and feed conversion ratio were estimated at 0.95 and 1.08% of diet, respectively, by broken-line linear model. Digestible Lys need for body weight gain was optimized at 1.05% of diet using broken-line quadratic model. In Cobb 500, Lys requirements for body weight gain and feed conversion ratio were estimated at 0.89 and 0.98% of diet, respectively, by broken-line linear model. Digestible Lys need for body weight gain and feed conversion ratio were optimized at 0.99 and 1.14% of diet using broken-line quadratic model.