This study was aimed to evaluate the effect of L-ascorbyl-2-polyphosphate as a dietary ascorbic acid source on blood parameters of Caspian brown trout (Salmo trutta caspius), including red blood cell (RBC) count, white blood cell (WBC) and WBC differential. A total number of 600 Caspian brown trout (9.6±0.6 g) fingerlings were randomly distributed in triplicates among five treatments each containing 40 specimens. Experimental diets were also prepared by adding 0, 50, 100, 200 and 400 mg kg^-1 L-Ascorbyl-2-Polyphosphate to the basal diet. Feeding was done for nine weeks in each treatment. The survival rate in all treatments was 100%. The results showed a significant increase in RBC, hemoglobin, hematocrit, WBC and lymphocyte (p<0.05) by supplementing L-ascorbyl-2-polyphosphate compared to the control treatment. The fish fed by 200 mg ascorbic acid kg^-1 diet had the maximum hemoglobin and hematocrit in comparison with the other treatments. The current research showed that dietary L-ascorbyl-2-polyphosphate influences the complete blood count of Caspian brown trout while fingerlings fed with the optimum amounts of 200 mg ascorbic acid kg^-1 diet for a period of 9 weeks trail.