A total of 324 one-day-old Ross 308 mixed-sex broiler chicks with an average initial body weight of 38 g were used in a 35-d growth assay to determine the effect of supplementing diets with lactulose on growth performance, carcass characteristics, fecal microbiota, and blood constituents. There were 18 birds/pen and 6 pens/treatment with food and water consumed ad libitum. Treatments were a corn-soybean-meal-based diet with none, 0.25 and 0.5% of lactulose. Data were analyzed as a completely randomized design using the mixed procedures of SAS (SAS Institute, 1996), orthogonal polynomial contrasts were used to look for linear and quadratic effects of Lactulose dose in the diet. Results indicated that gain of body weight (BWG; 1,715 vs. 1,765 vs. 1,843 g/d) and feed conversion ratio (FCR; 1.63 vs. 1.58 vs. 1.54) were improved linearly (P < 0.05). As for results increasing concentration of lactulose in the diet from none to 0.5% increased (P < 0.05) BWG and decreased FCR for d 8 to 21, 21 to 35, and 0 to 35. Also chickens fed supplemented diet with 0.5% lactulose showed higher relative weight of breast meat compared with other groups (7.26 vs. 8.50 vs. 8.64%). Inclusion of lactulose decreased the count of Salmonella (2.73 vs. 2.50 vs. 2.47 log10cfu/g) and E. coli (6.57 vs. 6.39 vs. 6.40 log10cfu/g) of feces in chickens fed diets containing 0.25 and 0.5% lactulose (P < 0.05), but the count of Lactobacillus was not affected. Addition of 0.5% lactulose decreased drip loss percentage on d 1 (P < 0.05; 3.98 vs. 2.94 vs. 2.35%) but there wasn’t any significant effect on meat color. Blood characteristics were not influenced. Thus it was concluded that inclusion of lactulose improved growth performance and altered fecal microbial populations with no negative effect meat quality and blood profile.