BACKGROUND: Carnitine biosynthesis needs methionine and lysine amino acids, vitamins niacin and ascorbic acid. So the usage of both L-carnitine and vitamin C may  improve the effects of only L-carnitine. OBJECTIVES: This study was carried out to investigate the effects of different levels of L-carnitine and vitamin C on the growth performance‚ body composition; blood serum parameters in Rainbow trout (Oncorhynchus mykiss) were investigated. METHODS: For this purpose, 540 fingerlings with the average weight 40±2 g were arranged in treatments including 3 vitamin C levels: zero, 500, 1000 and 2 L-carnitine levels: zero, 800 (mg/kg diet), with 3 replicates. RESULTS: Results showed weight gain, special growth rate and feed conversion ratio in the level of 500/800 vitamin C/L-carnitine had a significant increase compared to control (p<0.05). The highest and the lowest lipid content were observed in control and 1000/800vitamin C/ L-carnitine, respectively (p<0.05). The highest protein content was observed in 500/800 vitamin C/L-carnitine (p<0.05). Cholesterol and glucose showed a significant decrease in 500/800 vitamin C/L-carnitine. It was observed that 1000/800 vitamin C/L-carnitine caused significant decrease in blood triglycerides (p<0.05). CONCLUSIONS: Finally, it was determined that using vitamin C 500 and L-carnitine 800 mg/kg diet caused some improvement in growth performance with no  negative effects on blood parameters.