In this study, effects of a commercial trout diet (diet A) and a diet (diet B) made in a operation on growth of rainbow trout (Oncorhynchus mykiss) and reduce the cost of diet were investigated. The study was conducted in two periods: in the first period 1500 fish with an average weight of 23 g were stocked in each pond. The two experimental groups were fed with Diet A(I. group) and Diet B (II. group). At the end of the 60 days period, the average weight of the groups were 90.69 ±0.33 and 88.73 ±0.26 g and there was a significant differences between groups (p0.05). In the second period, 400 fish with average weights of 90.70 ±0.24 and 88.69±0.23 g were stocked in each pond. At the end of the 60 days period, the average weights of the groups were 26.11 ±1.62 and 237.55 ±1.54 g and there was a significant differences between groups (p0.05). At the and of the first period, feed conversion rate, specific growth rate and protein efficiency ratio were 0.99 and 0.99, 2.24 and 2.19, 2.02 and 1.96 while at the and of the second period they were 1.18 and 1.21, 1.49 and 1.64, 1.68 and 1.64 respectively in two groups. Atthe end of the total of 120 days, nitrogen retention and nitrogen excretion were 38.58 % and 36.36%, 61.41% and 63.64%, respectively in the groups. Because of the low differences in average weight of the fish at the end of the first period and 43.37% profit of diet B comparing with diet A, it was shown that, diet B could be used for fish of 23-90 g.