The objective of the current study was to evaluate the effects of a commercial prebiotic (Hemicell ®) as a growth promoter on growth performance, intestinal morphometry and IL-1 and IL-2 gene expression in spleen of Japanese quail. A total number of 360 one-day-old Japanese quail chicks were randomly allocated into 4 equal groups. Group 1(control group) fed on the basal diet that not supplemented with prebiotic, group 2 fed on basal diet supplemented with 0.5 g Hemicell/kg diet, group 3 fed on basal diet supplemented with 1.0 g Hemicell/kg diet and group 4 fed on basal diet supplemented with 1.5 g Hemicell/kg diet. To evaluate growth parameters, 30 birds from each group were weighed weekly for six successive weeks. Spleen tissues were collected at 21 and 42 days after supplementation from all quail groups (control and experimental groups). At the end of the experiment, the results of the current study revealed that, feeding Japanese quails on diet supplemented with Hemicell resulted in improved growth and growth performance as indicated by increased body weight and weight gain. Also, intestinal morphometry revealed an increase in height and base width of intestinal villi in a dose dependent manner. Moreover, Real time PCR indicated upregulation of the IL-1 and IL-2 genes expression in a dose and time dependent manner. In conclusion, addition of β-mannanase in Japanese quail diets would have beneficial effects on performance and immunity in birds.