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The aim for the present study was to investigate the effects of olive leaf (Olea europea L.) extract (OLE) on the control of Yersinia ruckeri infection in rainbow (Oncorhynchus mykiss) trout and to assess the impact on the expression of immune-related genes in the spleen and serum biochemical parameters of rainbow trout. Five experimental diets were prepared by adding 0.0%, 0.1%, 0.25%, 0.50% and 1.0% of OLE. Each diet was fed to triplicate groups of fish (mean body weight 51.22±3.04g) twice a day (at 09:00 and 17:00h) for 60 days. The dietary supplementation of OLE did not affect growth performance and feed utilization (P>0.05). Major changes due to graded levels of OLE in the diets were observed in blood biochemical parameters (P<0.05). TNFα, IL1-β and IL-8 gene expressions were significanlty up-regulated in OLE 0.1% group compared with others (P<0.05). Also, diet supplemented with OLE reduced mortality in rainbow trout fed with OLE 0.1% added diet. Present study suggests that OLE especially at 0.1% added feed may effectivelly enhance the serum biochemical parameters, survival rate and immune gene expression in rainbow trout.An nature plant extract show enhancement on immune responce in rainbow trout.Increasing disease resistant against fish pathogen.Short term application of olive leaf extract gave a valuable result for fish farms.This study result obtained in natural enviromental condition in a trout farm.