The effects of stocking density and dietary lysine level on growth performance, carcass composition and health status of separate-sex reared broilers were investigated. Using a 2×2×3 three-factor completely randomized design, 1 314 male and 1 530 female Ross 308 chicks were divided into 12 treatments with 6 replicates (pens) per treatment, respectively. Two stocking densities were designed in this experiment: 26 (low stocking density, 10 males/m2 or 12 females/m2) and 42 kg market weight/m2 (high stocking density, 16 males/m2 or 18 females/m2), respectively. A series of experimental diets with a 20% difference in lysine level (120%, 100%, and 80% of NRC recommendation standard) were compared in a 3-phase feeding program. The results showed as follows: high stocking density significantly decreased average daily gain (ADG) (P<0.01), and significantly decreased feed to gain ratio (F/G, P<0.01). On 1 to 35 days of age, high stocking density significantly decreased average daily feed intake (ADFI, P<0.01), however, the results were the opposite on 36 to 42 days of age (P<0.01). Male broilers had significantly higher ADFI and ADG, and lower F/G compared with female broilers on all feeding phases (P<0.01). On 1 to 21 and 1 to 42 days of age, sex and dietary lysine level had a significant interaction on ADG (P<0.05); on 36 to 42 days of age, sex and stocking density had a significant interaction on ADG and F/G (P<0.05). On 1 to 21 days of age, the ADG of male broilers was more severely affected by high stocking density. In contrast, female broilers were more severely affected in ADG and F/G by high stocking density from 36 to 42 days of age. The breast muscle ratio of broilers was significantly decreased by high stocking density (P<0.05) and low lysine diet (P<0.01). Female broilers had significantly higher breast muscle ratio and abdominal fat ratio compared with male broilers (P<0.01). High stocking density significantly increased the scores of gait and footpad burn, and the gait score of male broilers were significantly higher than that of female broilers (P<0.05). These results suggest that lysine requirement of broilers is not altered by stocking density, and the deteriorated effects of high stocking density are sex and age dependent.［Chinese Journal of Animal Nutrition，2011，23（4）：578 -588］