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The current research aimed at assessing the effects of different levels of Thymol on blood, feeding and survival factors in Oncorhinchus mykiss against Yersinia ruckeri. In this order, three experimental treatments including control (fed with commercial food), first treatment (commercial food with 0.5 percent Thymol) and second treatment (commercial food with one percent Thymol) were used with three replications for each treatment within a factorial analysis for completely randomized design. Then, fish were fed for seven weeks using associated diets. The result showed that different levels of Thymol had no significant effect on survival rate of Oncorhinchus mykiss (P>0.05). Regarding blood indices before the challenge, we found that the 0.5 percent Thymol had no significant effect, however, the one percent Thymol treatment showed significant difference (P<0.05) which caused an increase in the amount of hemoglobin and the concentration of intraglobulin hemoglobin. Measuring blood indices after the challenge, however, showed no significant different. The comparison of growth indices among different Thymol treatments showed that the 0.5 percent Thymol had significant effect on final weight, weight gain, special growth rate, protein yield ratio and food conversion factor compared to the control and one percent Thymol treatments. The result from current study showed that Thymol stimulates growth and improves nutritional conditions in fish, especially rainbow trout. [ABSTRACT FROM AUTHOR]