The present study throw lights on immunological aspects of Monosex Nile tilapia (Oreochromis niloticus) induced by using 17 α methyltestosterone (17-α-MT). A total number of 2000 yolk sac larvae were divided into two groups based on the exposure time (15 and 30 days). Each group was subdivided into three groups based on the concentration of 17-α-MT in the feed, first (control) group fed basal diet 0 mg 17-α-MT /kg food), group 2 (fed 40 mg 17-α-MT /kg food) and group 3 (fed 60 mg 17-α-MT /kg food). Fish in all groups were cultured under similar feeding regime and stocking density for six months. At the 5th, 5.5 th and 6th month of age, blood and tissue samples were collected for analysis of phagocytic activity of whole blood, lysosomal activity of plasma, plasma IgM, and the relative expression of IL-1β and TGF1β genes. The results showed that 17-α-MT treated groups were significantly higher in the phagocytic activity as compared with that of the control groups at the different experimental ages. While, treatment with 17-α-MT caused no significant variation in the phagocytic index, plasma lysozyme and IgM concentrations. MT treated groups showed a significant (P≤0.05) down regulation of IL-1β and up regulation of TGF1β gene expression level in head kidney and spleen when compared with those of the control group at the different experimental ages.