The present study was conducted to investigate the effect of basal diet type and emulsifiers on performance, immunological responses and ileal nutrient digestibility in broiler chicks fed diets with different fat sources. A total of 576 day-old Ross 308 broiler chicks were randomly distributed between 12 experimental treatments with 4 replicates of 12 birds each. Dietary treatments consisted of three different fat sources (corn oil, soy fatty acids and palm oil powder), two levels of an emulsifier supplement (0 and 0.1% of diet) and two basal diets (corn-soybean meal and wheat-corn-soybean meal), which fed to the birds according to a 3×2×2 factorial arrangement of treatments during a 42 d feeding trial. Performance parameters including average daily gain (ADG), average daily feed intake (ADFI) and feed conversion ratio (FCR) were measured periodically as 1-14 (starter), 15-28 (grower) and 29-42 (finisher) d periods. Serum samples were taken after immunization against Newcastle (NDV), infectious bronchitis (IBV), and Gumboro disease viruses to measure antibody responses against the mentioned antigens. In addition, two birds from each replicate were injected intraperitoneally with 0.5 cc 5% suspension of sheep red blood cells (SRBC) at d 24 and 32, and sera samples were collected at 7 th d after each antigen inoculation to evaluate primary and secondary antibody responses to SRBC. At 42 d of age, two randomly-selected birds per replicate were slaughtered and relative weights of internal organs and carcass characteristics were measured. The birds were fed with marker-containing diets during the last 3 d of trial and after euthanasia, the ileal digesta were collected for investigation of ileal nutrient digestibility. Results showed that ADG was higher (P 0.001) in chicks fed on wheat-containing diets than those on corn-soybean meal diets in starter period. Also, ADG of chicks fed on diets containing soy fatty acids were higher (P 0.001) than that of chicks fed diets containing soybean oil or palm fat powder in grower period. Except of starter period, dietary supplementation of emulsifier increased (P 0.05) ADG and consequently improved (P 0.05) FCR values throughout the trial period. Birds fed diets containing palm fat powder had the highest FCR values during starter (P 0.05), grower (P 0.001) and entire (P 0.05) trial periods. There was a significant (P 0.001) interaction between dietary fat sources and emulsifier supplementation in the starter period for ADG and FCR. Although emulsifier supplementation of diets had no marked effect on internal organ weights, caused an increase (P 0.05) in carcass percentage. The relative pancreas weight was lower (P 0.01) in chicks fed diets containing soy fatty acids than those on soybean oil- or palm fat powder-supplemented diets. Dietary fat sources did not affect serum triglycerides, but supplemental emulsifier caused a marked (P 0.05) increase in serum triglycerides concentration. In addition, dietary supplementation of emulsifier decreased (P 0.05) serum malondialdehyde content. The relative weights of bursa of Fabricius and thymus increased as the result of dietary emulsifier supplementation. Also, dietary inclusion of emulsifier improved NDV, Gumboro (P 0.05) and SRBC (P 0.01) antibody responses. Palm fat-containing diets decreased (P 0.01) Gumboro antibody titer. Dietary emulsifier supplementation increased (P 0.05) ileal fat digestibility, but had no obvious effect on digestibility coefficients of crude protein, total ash and organic matter. However, organic matter digestibility was lower (P 0.05) in the birds fed palm fat diets compared with those on fatty acid- or soy oil-supplemented diets. The present findings indicate that emulsifier supplementation of broiler diets can improve growth performance, and this effect depends on dietary fat sources, type of basal diet and bird's age. Furthermore, emulsifier supplement increased serum antioxidant capacity, which has a beneficial impact on bird health and carcass quality. Keywords : Broiler chicks, Dietary fat sources, Emulsifier, Ileal nutrient digestibility, Performance, Immune responses, Blood antioxidant index