This study aimed to investigate the effect of commercial phytase (SMIZYME PHYTASE) between diets containing 20 and 40 percent soybean meal on growth and body composition of rainbow trout. For this experiment, 300 Rainbow trout with an average initial weight of 6.5 ± 0.43 g were randomly distributed in 18 aquariums (15 fish per tank). Three replicates per treatment were considered. The fish were fed for eight weeks at a rate of 3% of body weight. To analyze the carcass some fish were randomly sampled and for growth characteristics, all fish from each tank were used at the beginning and end of the experiment. The result of this study indicated that phytase enzyme had a significant increase on some growth indices (p <0.05). Also in each levels of soy bean meal, adding 1,000 and 2,000 units of phytase could significantly increase protein content of carcasses. Adding commercial phytase to diet containing higher levels of plant protein with 40% soy replacement increased phosphorus, calcium and magnesium of carcasses. With increasing the level of phytase, the results of soy bean meal replacement is better and using 2,000 units of phytase per kg diet and 40% soy bean meal is recommended.