The present study was conducted to investigate the interactive effect of dietary methionine (Met) level and fat source on performance, blood biochemical parameters, carcass characteristics and immunological responses in broiler chicks. A total of 468 day-old Ross 308 broiler chicks were randomly distributed between 4 replicate pens (13 chicks per each) of each of 9 experimental treatments. Dietary treatments included three dietary fat sources (soybean oil, soy fatty acids and palm fat powder) and three different Met levels (Ross recommendations and 0.05% lower or greater than catalog specifications), that fed to the birds based on a 3 × 3 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 recorded periodically as 1-14 (starter), 15-28 (grower) and 29-42 (finisher) d of age. Serum samples were taken after immunization against Newcastle (NDV), infectious bronchitis, influenza and Gumboro disease viruses as well as sheep red blood cell (SRBC) to measure antibody responses. Furthermore, at d 42 of age two randomly-selected birds per replicate were slaughtered to measure relative weights of internal and lymphoid organs, and carcass characteristics. The results showed that there was significant difference among dietary fat sources for ADG during all trial periods, so that the highest ADG during the starter (P 0.01) and entire (P 0.05) trial periods were assigned to the birds fed on soybean oil- and fatty acids-containing diets, while at the finisher stage, the birds on soybean oil-diets had the higher (P 0.001) ADG compared with two other dietary fat sources. Dietary supplementation of Met at the recommended level improved (P 0.01) ADG throughout the trial period. Increasing Met level in diets containing soybean oil and fatty acids caused more increases in ADG compared with palm fat-supplemented diets, resulted significant (P 0.05) dietary fat source by Met level interaction. Although ADFI was not affected by dietary fat source or Met level, dietary supplementation with Met at the levels of Ross recommendation and 0.05% greater improved (P 0.01) FCR values during starter, grower and entire trial periods. Among the internal organ weights, only the relative liver weight was influenced (P 0.05) by dietary fat sources, so that dietary supplementation with palm fat powder decreased liver weight. Increases in dietary Met level by 0.05 and 0.1% relative to basal level increased (P 0.01) breast meat yield. In addition, incremental levels of Met were associated with increase (P 0.05) in relative spleen weight. Although increase in Met level by 0.05% relative to Ross recommendations increased thymus weight in soybean oil-containing diets, the opposite trend was seen with two other fat sources. The best antibody responses against SRBC (P 0.05) and NDV (P 0.01) during primary response were observed with soybean oil-diets. Increase in dietary Met level improved antibody production titer against Gumboro disease virus. Dietary inclusion of palm fat powder decreased serum concentrations of triglycerides (P 0.05) and HDL (P 0.001), while caused a significant (P 0.001) increase in serum cholesterol level. In addition, increasing dietary Met level decreased serum LDL and increased serum HDL concentration (P 0.05). Supplementation of diets with soybean oil and fatty acids increased (P 0.001) serum concentration of malondialdehyde (MDA), however, dietary Met level had no marked effect on serum MDA and enzyme activities related to liver health. In conclusion, the present findings indicated that dietary fat source could affect Met requirements, so that utilization of soybean oil and soy fatty acids increased Met demands to achieve optimum growth performance in broiler chicks. Keywords: Broiler chicks, Dietary fat source, Methionine requirements, Immune responses, Performance, Blood antioxidant capacity