In this study, rainbow trout diet was supplemented with different levels of annatto (Bixa orellana) seed powder and then some blood parameters (i.e. red and white blood cells count, hemoglobin concentration and hematocrit percentage), growth performance and the fillets color was evaluated. The experimental diets consisted of 0 (control), 0.25, 0.5 and 0.75% annatto powder with three replicates. A total of 360 juvenile rainbow trout (100±5 g) were stocked completely randomly in 12 tanks and reared for 40 days. The maximum amount of all measured blood parameters except white blood cells were obtained in supplemented diet with 0.75% annatto seed powder and the minimum amount of the blood indices were observed in control group (p<0.05). Also, the highest body weight (235 g) was related to 0.75% annatto seed powder and the lowest (108.2 g) was related to control one (p<0.05). Spectrum of red and yellow colors in the fish fillets were increased by increasing in annatto seed powder level in the diet and the highest values were recorded in the fish fillet fed with 0.75% annatto seed powder in comparison with others (p<0.05). Therefore, the addition of annatto seed powder at 0.75% in rainbow trout diet has a positive effect on the blood parameters, and on the other hand, by increasing the growth rate and fillet coloring, probably the cost of culturing will reduce and the customer's satisfaction with the fish meat will increase.