The present study was conducted to investigate the effect of incremental levels of dietary fat and methionine (Met) on performance, immune responses, blood biochemical parameters and ileal nutrient digestibility in laying hens. A total of 240 Hy-line W-36 laying hens of 46 weeks of age were used during a 77 days trial period. The birds were randomly distributed among 4 replicates (5 hens per each cage replicate) of each of 12 dietary treatments. Experimental treatments were consisted of 3 different soybean oil (0, 2 and 4% of diet) and 4 dietary Met (0.37, 0.41, 0.45 and 0.49 of diet) levels based on a 3×4 factorial arrangement of treatments. Performance parameters including feed intake, production percentage, egg weight, egg mass, feed conversion ratio (FCR) and egg quality indices were recorded periodically (two 35 d intervals). At the final trial day, two birds from each cage were slaughtered to measure relative weights of liver, gallbladder, pancreas, heart and spleen, as well as ileal contents. Antibody titers against Newcastledisease virus (NDV) and sheep red blood cell (SRBC) were measured after respective inoculations. In addition, sera samples were analyzed for lipid metabolites and malondialdehyde (MDA) concentrations. The results showed that the laying percentage at all trial periods were significantly (P 0.001) influenced by dietary fat levels so that dietary inclusion of 4% soy oil improved egg production. Also, dietary Met level of 0.49% increased hen-day egg production during the second 35d (P 0.01) and entire (P 0.05) trial period. Although dietary fat level had no marked effect on egg weight, increasing dietary Met level increased (P 0.05) egg weight during the entire experimental period. Dietary inclusion of 2 and 4% soybean oil, and consequently increasing dietary metabolizable energy level, decreased (P 0.05) feed intake. The best (P 0.01) FCR values were assigned to the birds fed on diets containing 4% soybean oil and 0.49% Met. Among the egg quality indices, shell breaking strength and thickness were affected by dietary fat level, so that using 2 and 4% soybean oil into the diets increased eggshell thickness (P 0.05) and strength (P 0.001) during the first 35d compared with control diet (without supplemental fat). Furthermore, increase in dietary Met level reduced (P 0.01) Haugh unit during the second 35 d. Increasing Met level up to 0.45% of diet improved the primary antibody response to SRBC (P 0.001) and NDVantibody titer (P 0.05). Supplementation of soybean oil into the dietsincreased antibody response against NDV at d 7 post vaccine inoculation, while decreased it at 14 th d after vaccination. Serum MDA content increased significantly (P 0.001) as the result of dietary soybean oil supplementation, however, incremental Met levelscould decrease (P 0.05) serum MDA content. Increase indietary Met level decreased (P 0.01) serum cholesterol, LDL and triglyceride concentrations. The results on ileal digestibility indicated that dietary inclusion of soybean oil improved digestibility coefficients of crude protein, lipid and dry matter. In addition, dietary Met levels of 0.45 and 0.49% increased crude protein and lipiddigestibility. The present findings show that increasing dietary fat level has an increasing effect on dietary Met demands, so that the highest Met levels in 4% soy oil-containing diets resulted in the best performance and productive efficiency in laying hens. Keywords: Laying hens, Methionine, Dietary fat level, Performance, Ileal nutrient digestibility, Immunological responses, Egg quality