A feeding trial was conducted to reduce the cost of diet by using two sources of soybean meal (conventional or genetically modified) instead of the expensive commercial fishmeal (FM). In the present study, commercial FM was substituted by partial (50%) or total (100%) soybean.  Diets were supplemented with formic acid (FA) at level of 0 or 10 ml/kg to investigate the growth performance and feed utilization of Oreochromis niloticus. The experiment was conducted with 360 Nile tilapia fingerlings (with initial weight, 22.15 ± 2.70 g). Nine isonitrogenous 25% crude protein and isocaloric 19.28 MJ/kg gross energy experimental diets were formulated. Over the 10-weeks feeding period, formic acid supplementation elevated the growth performance and feed utilization for fish fed either 50% conventional (C-SBM₅₀) or 50% genetically modified soybean meal (GM-SBM₅₀). Serum ALT, AST, glucose and triglyceride values were lower in fish fed the control diet and conventional soybean in comparison with those fed genetically modified soybean meal. No obvious changes were found in micronucleus or in nuclear abnormalities for all diets compared with the control diet. FA supplementation improves the growth performance, feed utilization and biological abdominal parameters in fish fed either C-SBM₅₀ or GM-SBM₅₀. In spite of that, genetically modified soybean elevated the levels of ALT, AST, glucose and triglyceride. These results draw our attention to the important use of organic acids in fish diets to improve the growth performance and feed utilization. As regarding the genetically modified soybean, it needs more investigation to evaluate their effect on feeding trial on fish.