As recently applicable, there are few studies on the impact of using nano-Se on varied fish species. Where nothing reachable focused on its impact on tilapias so, the present analysis evaluated the efficacy of using Nano-Se in tilapias on immune response, antioxidant defense compared by conventional Se form. 480 O. niloticus fingerlings were haphazardly grouped firstly into three groups with four replicate of each. The control one (CT) was fed on basal diet. The second and third one supplemented with 0.7 mg/kg Se and nano-Se respectively for nine weeks. At start day of the ninth week, two replicates from each group were injected by Streptococcus iniae where, the remaining replicates without challenge. Enhancement of growth performance measurements were noted in nano-Se compared to Se or CT groups. Existed anemia in S. iniae tilapias became alleviated by using nano-Se that also, improves the alteration of leucogram induced by challenge. Elevation of aminotransferases, alkaline phosphatase, lactate dehydrogenase (ALT, AST, ALP and LDH) and creatinine in Se and CT challenged replicates that seemed nearly normal by using nano-Se. using nano-Se showed more powerful antioxidant activities than Se uses. There were an expansion of immunoglobulin M, lysozymes, Glutathione peroxidase, nitric oxide, superoxide dismutase and catalase (IgM, LYZ, GPx, NO, SOD, CAT) and their related gene expression in nano-Se with contrast in Se or CT challenged groups. Nile tilapias challenged by S. iniae disclosed substantial expansion in the percentage of mortality in CT challenged fish (93.33%), followed by the group supplemented with Se (73.33%), whereas the lowermost one at fish supplemented by nano-Se (26.66%). The mortalities have been stopped from the 5th, 12th and 14th days in, nano-Se, Se and CT respectively. It can be concluded that using of Se 0.7 mg/kginduce immunosuppressive, antioxidant, liver and kidneys negative impact on tilapias where the same dose from nano-Se was more potent immunomodulating and antioxidant. Also it is attend in counteracting the serious impact induced by S. iniae challenge.