BACKGROUND: Replenishing fish oil with vegetable oils and using dietary supplements plays an important role in the metabolism of fats in aquatic animals. Corn oil is one of the vegetable oils that can be a good alternative to fish oil. OBJECTIVES: The aim of this study was to investigate the effects of replacing fish oil with corn oil and the use of dietary supplements L-carnitine and betaine on the metabolism of fats and blood indices of rainbow trout. METHODS: 450 rainbow trout (9.12±0.26 g) were divided into four treatments (three replicates) and were fed with manual diet containing fish oil (1st treatment), corn oil (2nd treatment), fish oil with 500 mg/kg of food containing L-carnitine and betaine (3rd treatment) and corn oil with 500 mg/kg of food containing L-carnitine and betaine (4th treatment) for eight weeks. At the end of study, fatty acid profiles of the treatments were determined by gas chromatography (GC) and blood indices and their weights were also examined. RESULTS: At the end of the experiment, the weight index did not show a significant difference. In the replacement of fish oil with corn oil, the levels of PUFA n-3, C18:3n3, C20:3n3, C20:5n3 and C22:6n3 fatty acids significantly decreased in fish carcasses and the PUFA n-6, C18:2n6, C20:2n6 and C20:4n6 increased significantly, but no significant difference was observed in total SFA and MUFA fatty acids. L-carnitine and betaine increased the EPA accumulation in fish oil and corn oil significantly, and the number of white blood cells in corn oil, L-carnitine and betaine fed fish (fourth treatment) were increased compared to fish oil treatment (first treatment), significantly (p < /em><0.05). CONCLUSIONS: Considering the effect of corn oil, L-carnitine and betaine supplements on increasing the n-6 PUFA fatty acids, essential EPA and white blood cell counts, the use of corn oil and L-carnitine and betaine supplements in rainbow trout diet is recommended.