The fishing industry is deeply dependent on complex and high quality diets consisting of dry feed, vitamins and micro/macro-elements. The suitable dose of each essential nutrient is different based on the fish species, their age and physiological conditions. In this study, the effects of two recommended doses of riboflavin (20 and 30 mg vitamin per each kg dry feed) on growth, hematological and immunological parameters of rainbow trout (Oncorhynchus mykiss) fingerlings were investigated. The results showed that although treatment with riboflavin could improve biometric and immunological parameters of the fishes but the higher concentration (30mg) of riboflavin had no extra-positive effects on these factors. Furthermore, both doses decreased hematopoietic activity of this fish. Overall, the results of this study showed that treatment with 20 mg of riboflavin is more effective than treatment with 30 mg of riboflavin per kg of dried food.