The aim of this research was to evaluate the effects of oral recombinant bovine somatotropin (RBS) 1% on growth performance of rainbow trout (Oncorhynchus mykiss). 600 rainbow trout weighing 46±1 grams were randomly allotted in two groups including: 1) the control group and 2) RBS treated group, each in three replicates. The fish were hand-fed with commercial pellets. The fish received 200g RBS 1% per 100kg BW. The administration method of RBS-mixed feed to fish was once a week for 4 consecutive weeks, totally 8 times for 12 weeks of rearing periods. At the end of each rearing periods (on day 28, 56, 84) the means of some of growth characteristics were calculated in the control and RBS treated groups. The obtained results in this research demonstrated that there were no significant differences between appearance growth characteristics in the control and RBS treated groups on days 28 and 56 of rearing periods (p> 0.05). However, there were significant differences between means of whole body weight (p < 0.0 5), weight gain (p < 0.05), average daily growth ( p < 0.05) , total length ( p < 0.001) , and average daily length ( p < 0.05 ) of the control and RBS treated groups on day 84 of rearing period. In the same period, there was greater 11.2% comparative growth rate in RBS treated group than the control group. Generally, the obtained data showed that oral administration of RBS 1% can produce a significant increase in the growth rate of rainbow trout weighing 46±1 grams on day 84 of rearing period.