This study was conducted to evaluate the effects of alfalfa (Medicago sativa) extract on growth performance, feed intake, body composition and some of the serum parameters of Rainbow trout (Oncorhynchus mykiss). In this study, a total of 240 rainbow trout were weighted average 8/36±0.2(g) and randomly distributed in four treatments (control), 0.5, 1, 1.5% of extract per (kg) of diet with three replications in plastic tanks. Fish were placed and fed with extract containing food for 60 days. During this time, physicochemical factors of water were measured. At the end of the period, growth index and serum parameters calculated. The results showed that growth performance in treatments that fed with 1% alfalfa extract had a significant difference compared to control treatment (p<0/05). In the indexes of body compositions, an increase in the amount of fat was observed in 1% treatment of the extract, while there was a significant difference with the control group (p<0/05). Serum parameters showed that there were significant differences between fish fed with alcoholic extract of alfalfa and control group (p<0/05). The lowest cholesterol and triglyceride levels and the highest level of protein were observed in the 1.5% extract group. Also, the highest amount of glucose was observed in control treatment. The results of this study showed that the addition of alcoholic extract of alfalfa (1 -1.5 %) to fish diet can improve the growth performance and blood biochemical parameters of rainbow trout.