BACKGROUND: In recent years, use of herbs as dietary irritant growth factors and replacing it with artificial materials in aquaculture is discussed. Rosemary (Rosmarinus officinalis) oil is one of herbal medicines which its antioxidant and antibacterial properties have been proved in humans and animals. So it can be an alternative option in order to achieve artificial growth stimulants properties. OBJECTIVES: The purpose of this study was to investigate the effect of different levels of rosemary oil as a medicinal herb on growth parameters and gut morphology of beluga juveniles (Huso huso). METHODS: Six groups (three replicate/group) of 126 fish with initial mean body weight of 130.94 ± 5.28g were fed with diets containing 0 (control), 0/01, 0/1, 1 and 2 % of rosemary oil and a group of oxytetracyclin (30mg/kg) for 8 week. At the end of the trial, the growth parameters (final weight, specific growth rate and feed conversion ratio) were calculated. Then, beluga intestinal biopsy was performed and the mean villi length, width and surface were measured in the treatments. RESULTS: Significant increase were observed in villi length (2/25±0/11mm), width (0/52±0/01mm) and surface (1/17±0/07mm) in fish fed with diets containing 1ml/kg rosemary oil (p<0/05). There were no significant differences in growth parameters between treatments. CONCLUSIONS: According to the significant increase of beluga intestinal absorption area and slight improvement of growth parameters in fish fed with diets containing rosemary extract compared to the control group, it seems that rosemary oil can act as a growth stimulant. So it can be considered as an alternative to artificial growth stimulants in aquaculture industry.