BACKGROUND: Use of herbs in aquaculture has become more common and rainbow trout is one of the fish that is widely cultured in most countries. OBJECTIVES: The purpose of this study was to find the effect of Basil aqueous extract on growth, hematological and biochemical parameters in rainbow trout. METHODS: The experimental treatments were formed by five different diets including: diet without Basil aqueous extract (control), diet with 30, 60, 120, 240 ppm Basil aqueous extract per food. Rainbow trout with an average weight of 12±2.6 g were fed with these diets for 48 days and at the end of experiment, growth parameters (FCR, SGR) and blood parameters were measured. RESULTS: Evaluation of FCR and SGR showed that there is no significant difference between the control and treatment group (30, 60, 120 and 240 ppm). The result of hematological parameters were showed that basil aqueous extract increased RBC (1.24±0.04×106 mm3) in the 120 ppm group, and also increased WBC (22.7±3.4×103 mm3), Hemoglobin (6.3±0.6 g/dL) and the mean cell hemoglobin (57.1±3.8 pg), in the 240 ppm group. The result of biochemical parameters were showed that Basil aqueous extract decreased Cholesterol (163.7±20.15 mg/dL) and triglyceride (166.5±16.2 mg/dL) in the 30 ppm group.  On the other hand, there was no significant difference in Glucose in the treatment group compare to the control. CONCLUSIONS: Seems that adding Basil aqueous extract to rainbow trout diet may improve the positive physiological characteristics.