In this study, a total of 60 juvenile Acipenser persicus (mean weight 410±267.6, mean length 39.6±19.6 cm and 1 - 2 year old) were captured by trawling method at depths of 20 to 100 meters in the Mazandran province via 7 times surveys during 2009 – 2012, and studied immunologically. Results showed that the mean of red blood cells (RBC) and white blood cells (WBC) were 585000 ± 151000 and 30300 ± 3200 per mm3 of blood, respectively. Mean percentage of lymphocyte, monocyte, neutrophile and eozynophile were 83.1 ± 12, 2.9 ± 2, 8.3 ± 6.6 and 5.6 ± 3.2 %, respectively. Mean concentration of Immunoglobin M (IgM) in blood serum was 25.6 ± 8.7 mg/dl. Samples showed no significant difference in length (P>0/05). These results showed that the fish by less than the length classes were more sensitive to environmental changes. For this reason, their immune indices improved. Therefore, any change in fish blood immunological factors can lead to the secondary response.