The present study was carried out to evaluate the effects of various amounts of organic and mineral selenium supplementation on growth performance (including condition factor (CF), specific growth rate (SGR), and weight gain (WG)), heamatological indices and liver histopathology. Nine dietary treatments were prepared and supplemented with selenium at 0 (control group (T0)), 0.15 (T1), 0.3 (T2), 0.45 (T3) and 0.6 g/kg of fish diet (T4). Fish were fed with formulated diet during 60 days of experiment. The average weight gain of the T3 group that was fed with organic selenium (214±18.6 g) was significantly higher (P<0.05) than the control group (154/6±25 g). The average SGR values in T2, T3 and T4 that were fed with organic selenium and the group T2 that was fed with mineral selenium were significantly higher than the control group (P<0.05). The concentration of IgM was significantly higher (P<0.05) in T1 (55.05±11.6 mg/dL) that was fed with organic selenium as compared to control group (46.9±6.6 mg/dL). Lysozyme activity in T3 (1066.7±24.5 unit/g) that was fed with organic selenium was significantly higher (P<0.05) than the control group (838.5±11 unit/g). The highest amounts of C3 (34.8±28.4 mg/dL) and C4 (31.1±7.3 mg/dL) were observed in T4 group that was fed with organic selenium. These results indicated that dietary supplementation of organic selenium up to 0.45 mg/kg of fish diet represented an appropriate ratio of growth factors, immune responses and liver pathological properties in rainbow trout.