The present study was conducted to investigate the effect of dietary supplementation of organic acids on performance, immunological responses, ileal nutrient digestibility and gut microbial populations in broiler chicks fed diets with different cereal sources. A total of 450 day-old Ross 308 broiler chicks were randomly distributed among 5 pen replicates of each of 9 experimental treatments according to a 3 × 3 factorial arrangement of treatments. Dietary treatments included three different basal diets (diets based on corn-soybean meal, and diets containing wheat or barley) and three status of organic acid supplementation (control, and 0.25% of butyric or citric acids), that fed to the birds during a 42 d feeding trial. At d 11 and 27 of age, the birds were vaccinated against Newcastle disease virus (NDV) and sera samples were collected 7 d after each inoculation. In addition, the chicks were vaccinated for Gumboro and infectious bronchitis (IBV) viruses. On d 27 and 33 of age, sheep red blood cell (SRBC) was injected and sera were collected at d 7 postinoculations. To investigate the ileal nutrient digestibility, the birds were fed with marker-containing diets (0.5% of diet, Celite) for 4 d form 39 to 42 d of age. At d 42, two birds from each replicate were selected on random, thereafter, weighed and slaughtered. After measuring internal organ weights, the ileal content was collected for ileal digestibility and microbial count. The results showed that except finisher period, dietary supplementation of organic acids increased (P 0.05) body weight gain. Diets based on corn-soybean meal increased weight gain in starter and finisher periods; in grower stage, however, diets containing wheat and barley improved (P 0.05) daily weight gains. Dietary inclusion of acidifiers had no marked effect on feed intake. Daily feed consumption was not affected by basal diets during starter and grower periods, however, using corn and barley diets increased feed intake during finisher (P 0.001) and entire (P 0.05) trial periods compared with wheat-containing diets. Dietary supplementation of organic acids caused considerable improvements in feed conversion ratio in grower (P 0.001) and entire (P 0.01) trial periods. Dietary inclusion of both acidifiers decreased (P 0.05) pancreas and ceca weights; however, only citric acid decreased (P 0.001) the relative gizzard weight. Diets based on corn and barley caused significant decrease in liver weight and increase in pancreas weight (P 0.05). Dietary inclusion of barley increased the relative weights of ceca (P 0.05) and gizzard (P 0.01). The highest (P 0.05) breast meat yield assigned to the birds on butyric acid-supplemented diets and utilization of corn-soybean meal-based diets caused numerical increase in breast and carcass yields. Although the relative weights of lymphoid organs were not influenced by dietary treatments, acidifiers improved (P 0.05) antibody titers against NDV, IBV and SRBC. Dietary supplementation of organic acids increased (P = 0.058) serum triglyceride concentration. Furthermore, diets based on wheat and barley decreased serum concentrations of LDL (P 0.01) and cholesterol (P 0.05), and increased (P 0.01) serum HDL level. Results on ileal digestibility showed that dietary supplementation with both acidifiers improved (P 0.05) ileal digestibility of dry and organic matters. In addition, feeding corn-soybean meal diets increased (P 0.05) fat and dry matter digestibility. Dietary supplementation of both organic acids decreased (P 0.001) pathogenic bacteria in ileum content, while barley and wheat-containing diets increased (P 0.05) these microbial populations. The present findings indicate that dietary supplementation of organic acids could increase the ability for barley and wheat utilization in broiler diets. In addition, organic acids had beneficial impact on feed efficiency and carcass yield and modulated intestinal bacterial populations. Keywords: Broiler chicks, Butyric and citric acids, Non-starch polysaccharides, Performance, Immune responses, Ileal nutrient digestibility, Ileum microbial population