Abstract: In this study, Bacillus subtilis is used for control of streptococcosis with the agent of Streptococcus iniae in rainbow trout. The experience was carried out in 2 groups (Control (c) and Treatment (T)) and 3 replicates. In control group, probiotic was not applied in diet but in treatment group, Bacillus subtilis was administered in feed at a concentration of 107 cells g-1. At 45th day, 0.1 mL intraperitoneally injection of S. iniae with 2x107 cells mL-1 dosage was given to both groups and were checked for the rest of the survey duration (2 weeks). At the end of the time which took about two months, blood samples were caught for hematological experiments for realizing the effect of B. subtilis feeding on resistance of fish against S. iniae infection. After injection of S. iniae, there was no significant difference among T and C groups considering parameters such as erythrocyte count, hemoglobin, hematocrit, Mean Corpuscular Volume (MCV), Mean Corpuscular Hemoglobin (MCH), Mean Corpuscular Hemoglobin Concentration (MCHC) and percent of monocyte (p>0.05). But significant difference was seen in the leucocyte count, percent of lymphocyte and neutrophile in both groups (p<0.05). The leucocyte count and percent of lymphocyte was higher in T group and neutrophile percent was lower in comparison with the control. The results of the present study indicate that B. subtilis can be used as an agent for the control of streptococcosis in rainbow trout hatchery and culture farms for decreasing economical disasters.