This study was performed in order to determine the effect of Oregano (Origanum vulgare) extract on Streptococcus ineae infection in rainbow trout. 360  rainbow trout weighing 15±1 gr were randomly allocated into three treatment groups with three repetition,  including: 1) Control group, 2) Oregano extract treated group (1% of diet), 3) Felorfenicol treated group (10mg/kg fish), all feed one time a day for two weeks. At the end of the second week, 5 fish of each repetition collected and sampled from their blood and tissue of spleen and kidney. After the bleeding, all the samples injected with streptococcus ineae. After two weeks mortality rate calculated and after the bleeding, sampled from the spleen and kidney of the samples. Total cunt analysis conducted on the spleen samples in 4 logarithmic concentrations. in the second phase all the samples primarily injected with S.ineae and then treated with Oregano concentration and Felorfenicol for two weeks. After the two steps of bleeding, in sterile qualifications, the spleen of the fishes sampled for total cunt analysis. Blood cells counting and lisosym activity indicated in all blood samples. The results shown that total mortality in both of the phases of experiment in Oregano extract (OE) treatment was significantly lower comparing to control group (p<0.05). Furthermore the results of blood cells counting shown that WBC and monocyts percents were significantly higher in Oe and Felorfenicol treatment groups (p<0.05). This phenomenon was because of the effects of Oe and Felorfenicol. Nevertheless the lysozym activity degreased comparing to control group (p<0.05). But total count analysis shown that bacterial audience in spleen degreased treating by Oe and felorfenicol. It was concluded that supplementation of Oe registered lower mortality and bacterial audience, and better hematological parameters in fishes, compared with control group. Therefore supplementation of Oe in fish diet would be effective in prevention and treatment of S.iniae in rainbow trout.