The objective of this study was to evaluate the influence of supplementing betaine to broiler diets containing adequate concentration of methionine on growth performance,  carcass traits and some blood parameters, as well as economic efficiency. Two hundred and sixty  day-old Ross broiler chicks were allotted into five equal treatments of four equal replications. Five experimental isonitrogenous and isocaloric diets with graded levels of betaine (0.0, 1.0, 1.5, 2.0 and 2.5 g/kg diet) were formulated and fed to chicks from 1 to 39 days of age. During the entire experimental period, dietary betaine supplementation had positive effects on mortality rate, economic efficiency and growth performance (weight gain, feed conversion ratio, protein efficiency ratio, efficiency of energy utilization and performance index) of broiler chicks but feed intake was not affected. Broilers fed the betaine-supplemented diets exhibited significantly higher percentages of carcass yield, total edible parts, breast yield and breast meat while those of heart and abdominal fat were decreased but relative weights of giblets, liver and gizzard were not affected compared with their control counterparts. Feeding the betaine-supplemented diets also produced a positive effect on liver and breast meat composition (increased crude protein and decreased ether extract contents of both) compared with the control group. Dietary supplementation with betaine led to significant increases in serum concentrations of total protein, globulin, triglycerides and total cholesterol but levels of glucose and albumin were not affected. Taking the economic aspect into account, it is concluded that supplemental betaine can be used to improve growth performance and carcass quality of broilers fed methionine adequate diets. Based on the present results, a supplemental dietary betaine level of 1.0 g/kg diet is suggested to be enough to exert such beneficial effects.