The purpose of this study was to evaluate the effect of Butachlor on some blood parameters of rainbow trout (Oncorhynchus mykiss). 150 pieces of fish in five experimental groups (25, 50, 75, 100% dosage of LC₅₀ and control) were exposed to various concentrations of the herbicide Butachlor for 6 days and changes in the blood parameters were studied. At the end of experiment, there were significant difference (p <0.05) between experimental treatments in terms of their number of red and white blood cells, hemoglobin and hematocrit values. ​​MCV, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration. The number of red and white blood cells, hemoglobin and hematocrit values decreased with increasing Butachlor concentration. Also MCH and MCHC showed decreasing trend by increasing percentage of Butachlor. These two factors as well as MCH were significantly different in control compared to the other treatments. Regarding MCV, an insightful difference was observed between control group and other groups. Increasing Butachlor concentration reduced every studied factor eloquently. Moreover, in every factor except WBC, there was a significant difference between low ( 25% , 50%) and high (75% , 100%)concentration of the Butachlor. Evidently, change in different blood factors of fish under the Butachlor toxin had cause a reduction in their protection system and mortality.