The present study was designed to investigate the interactive effect of dietary organic acid (OA) supplementation and crude protein (CP) level on performance, blood parameters, immunity and gut morphology of broiler chickens. A total of 540 Ross 308 broiler chicks were randomly distributed into the 4 replicate pens of each of 9 dietary treatments according to a 3×3 factorial arrangement of treatments. Experimental diets consisted of three dietary CP levels (Ross values, and 1 and 2 percentage points lower than) and three situations of dietary organic acid supplementation (control group or groups supplemented with 0.25% of citric or butyric acids). Average daily gain (ADG), feed intake (ADFI) and feed conversion ratio (FCR) were recorded periodically as 1-14 (starter), 15-28 (grower) and 29-42 (finisher) d of age. At d 12 of age, birds were vaccinated against Newcastle disease virus (NDV); thereafter, the serum samples were taken to evaluate antibody production titers against NDV at d 6 and 12 after vaccine inoculation. Similarly, antibody responses against infectious bronchitis virus (IBV) and infectious bursal disease (IBD) were measured after respective antigen inoculations. At the final trial day, two randomly-selected birds per replicate were slaughtered to measure carcass characteristics. Also, sections of jejunal epithelial cells were embedded in 10% formalin solution to evaluate morphological alterations of absorptive cells in response to mentioned dietary modifications. Results showed that dietary supplementation of citric and butyric acids caused significant (P 0.01) increase in body weight gain during finisher period compared with birds on control diet; however, decrease in dietary CP level had no effect on ADG values. In addition, CP by OA interaction was shown to significantly affect ADG during both finisher and overall trial periods so that, dietary OA supplementation was more effective in diets contain in two higher CP levels. Although ADFI wasn’t influenced by experimental variables, but interactive effect of dietary CP and OA was significant for feed consumption during finisher and entire experimental periods. Dietary inclusion of organic acids especially butyrate caused marked (P 0.05) decrease in FCR values. Also, FCR was affected (P 0.05) by dietary CP level with the best values assigned to the birds fed on highest-CP diet. Decrease in dietary CP content was associated with decreases in relative weight of liver (P 0.05) and carcass percentage (P 0.01), and significant increase in abdominal fat. Similarly, OA supplementation reduced (P 0.01) liver weight and improved (P 0.05) carcass percentage. Interestingly, the relative weight of gizzard was reduced (P 0.01) by dietary OA supplementation. Dietary reduction of CP caused considerable (P 0.05) decrease in relative weight of bursa of Fabricius, while spleen weight was increased in chicks fed on OA-supplemented diets. The results demonstrated that serum triglycerides, high-density lipoproteins and low-density lipoproteins weren’t affected by OA supplementation or dietary CP level at d 21 of age. On the other hand, serum uric acid significantly (P 0.01) decreased with lowering dietary CP level at both d 21 and 42 of age. Antibody titers against NDV was significantly (P 0.01) affected by butyric acid-supplemented diets at d 12 post vaccine inoculation, however, this parameter wasn’t influenced by dietary CP level. Similarly, IBV and IBD antibody responses were significantly (P 0.05) improved by dietary OA supplementation. Although crypt depth wasn’t affected by dietary treatments, villi height to crypt depth ratio decreased by dietary CP reduction. From the present findings, it can be concluded that dietary supplementation of 0.25% citric and particularly butyric acid, could improve weight gain and feed efficiency, and dietary CP content could be reduced by minimum 1 percentage point via dietary addition of citrate or butyrate. Keywords: Broiler Chicks, Dietary Crude Protein, Organic Acid, Performance, Immune Responses