This experiment was conducted to determine the effects of the ratio of methionine plus cystine to lysine in diets on growth performance and economic benefits of broiler chickens, and to study the optimum (Met+Cys)/Lys ratio in the diet. A total of 3 600 one-day-old Ross 308 mix-sexed broilers were obtained from a commercial hatchery and randomly divided to 4 dietary treatments with 3 replicates per treatment and 300 broilers (half male, half female) per replicate. Graded levels of methionine were supplemented to obtain digestible (Met+Cys)/Lys ratios of 0.64, 0.68, 0.72 and 0.76 in the diet, respectively. Growth performance, plasma free amino acid concentrations and economic benefits of broiler chickens were determined in the trial. The trial lasted for 35 days. Results showed that dietary (Met+Cys)/Lys ratio had no significant effects on the growth performance of broilers (P>0.05). The feed/gain of broilers aged from 1 to 35 days was linearly increased (P=0.05), but the concentrations of methionine and cystine in plasma were linearly decreased (P<0.05), and the economic benefits were decreased when the dietary (Met+Cys)/Lys ratio decreased from 0.76 to 0.64. The concentrations of urea nitrogen, uric acid, serine and isoleucine in plasma of broilers were significantly increased (P<0.05), and the glycine concentration was linearly increased (P<0.05), but the tyrosine concentration was decreased in a quadratic curve with the decrease of dietary (Met+Cys)/Lys ratio (P=0.05). The concentrations of glutamic acid and ammonia in plasma of broilers were significantly increased when dietary (Met+Cys)/Lys ratio decreased to 0.68 (P<0.05). There were no significant differences in the concentrations of the other amino acids and total amino acids in plasma among all groups (P>0.05). Results suggest that the optimum dietary(Met+Cys)/Lys ratio is 0.76, in which growth performance of broilers is improved, plasma free amino acids are balanceable and economic benefits are better.