An experiment was conducted to determine the appropriate value of vitamin C on growth, survival, and Hepatosomatic Index (HSI) in Caspian brown trout (Salmo trutta caspius) fingerlings. Five diets containing 0, 50, 100, 200 and 400 mg kg^-1 ascorbic acid were prepared as experimental treatments. Six hundred fish (9.6±0.6 gr) were divided into 5 groups with three replications each containing 40 fingerlings and fed by the respective diets for 9 weeks. Growth parameters including Final Weight (FW), Protein Efficiency Rate (PER), Weight Gain (WG), Specific Growth Rate (SGR), Feed Conversion Rate (FCR), Condition Factor (CF), and HSI were compared every 3 other weeks between the treatments. Fish sampling was done weeks. The results showed that survival rate of the fish in all experimental treatments was 100% and no sign of ascorbic acid deficiency such as gill deformities, abnormal swimming, and skeletal deformities including scoliosis and lordosis were observed in all Caspian brown trout fingerlings during the experiment. Also there was significant differences in FW, WG, SGR, PER, FCR, CF and HSI between experimental treatments (P<0.05). The results showed that 200 mg kg^-1 ascorbic acid were significantly increased the growth parameters of Caspian brown trout (P<0.05), although no significant differences were observed in fish growth rates between 200 and 400 mg ascorbic acid treatments. Therefore, findings of the present study illustrated that 200 mg ascorbic acid per kg diet is the optimum amount of vitamin C for the growth of Caspian brown trout fingerlings. According to the broken line model, the level of ascorbic acid for optimal weight gain was 248.52 mg kg^-1 diet.