Rainbow trout is the most important species of cold water fishes in Iran and the feeding diets which improve the quality and growth of this fish are of great importance. Rainbow trout Oncorhynchus mykiss (n=360) specimens with approximate weight of 2±0.02g were fed with different concentrations of sesame seed in their diet instead of fish meal for 120 days. Feeding diets were prepared in 3 sesame seed concentration levels (10, 15, 20 percent compared with control (normal diet), completely random design was used for all triplicate experiments. The greatest rate of weight gain (74±0.04g.), length gain (18.5±0.01cm), specific growth rate (0.59), protein efficiency rate (2.55) and food conversion ratio (0.9) were obtained in treatment 3(20% sesame seed).The lowest rate of weight gain (59±0.04g.), length gain (16.5±0.01cm), specific growth rate (0.46%), protein efficiency (1.72%) and food conversion ratio (1.3) was obtained in the control treatment. Results indicated presence of highly significant differences among the treatments (p < 0.01). There was no significant difference between the composition of the carcasses of these fish fed with diet of 20 percent sesame and that of control treatment fish. These results demonstrated that sesame seed can be used as an alternative ingredient in its feeding diet instead of fish meal.