The aim of this study was to survey the effects of different levels of Stevia alcoholic extract on performance and humoral immune system of broilers. Two hundred one-day chicks (Ross 308) were studied in a completely randomized design with 5 treatments, 4 replications and 10 chicks per replicate. The treatment groups received 0 (control), 0.5, 1, 1.5 and 2 mL/L of Stevia extract, in drinking water, during days 3 to 42. Daily feed intake, daily body weight gain, feed conversion ratio and carcass characteristics were measured. The birds were immunized by sheep red blood cell (SRBC) on days 12 and 29 of age and serum antibody levels produced in response to SRBC were measured on days 28, 35 and 42. Newcastle vaccine was given on days 1, 6, 18 and 29 to chicks and blood samples were collected at 42d. Antibody titer against Newcastle virus was determined by the HI method. Result indicated that Stevia alcoholic extract hadn’t any significant effect on performance (P>0.05). All experimental groups increased antibody titer against Newcastle virus (P<0.05). Consumption of 1.5 and 2 mL Stevia extract increased anti SRBC titers (P<0.05). It is concluded that Stevia alcoholic extract had no significant effect on performance, but improved humoral immunity of broilers.