The effects dietary supplementation with Aloe vera extract (AE) on the immunity responses and hematological parameters of rainbow trout (Oncorhynchus mykiss) fry were evaluated in eight weeks trial. 600 rainbow trout (O. mykiss) fry with an average initial body weight of 13±0.05g were randomly allocated into two treatment groups including placebo-treated group (control) and A. vera extract-treated group, each with three replicates. The fish were hand-fed once a day with diet medicated AE or placebo (70 % lactose, 10 % starch and 20 % talc) at a rate of 1% of feed weight in the first feeding for 8 weeks. At the end of the identical every two weeks (2, 4, 6, and 8 weeks) 24 h after feeding, some of immunological and hematological parameters were analyzed. The results showed that serum total protein, albumin and globulin, respiratory burst activity, phagocytic activity and serum lysozyme activity vary among the two treatment groups which were found to be higher in the AE-treated group (p < /em><0.05). However, there were no significant differences in none of hematological parameters between two groups. It was concluded that supplementation of AE at a rate of 1% in feed registered higher immunological responses in compared to placebo group. Therefore, supplementation of AE in fish diet would enhance the immunity responses in fish. It may use in fish diets particularly at time of outbreaks.