Abstract: To study the comparative effects of Nano Selenium (Nano-Se) and sodium selenite on the growth, bioavailability, antioxidative activities, hematological and biochemical parameters, cellular and humoral immunity, a trial was conducted on BV 300 layer birds during grower phase (9-20 after weeks) in six treated groups. After twenth weeks body weight of the all Nano-Se treated groups (up to a dose of 0.3 mg kg^-1 of diet) was found to be significantly (p<0.05) higher than sodium selenite treated and control groups. However, further increase in dietary Nano-Se content in feed had negative effect on body weight of bird. Birds fed with both Nano-Se and sodium selenite showed higher (p<0.05) Se content in different tissues (Such as breast muscle, liver, kidney, pancreas, serum and feathers). However, Se content in liver, breast muscle, pancrease and feathers were signifcantly higher (p<0.05) in Nano-Se treated groups. In addition, significant (p<0.05) difference was observed as regard to glutathione peroxidase (GSH-Px), erythrocyte catalase (CAT) and superoxide dismutase (SOD) activities among the treated groups. Significantly better cellular and humoral immunity response were observed in Se supplemented birds. Dietary supplementation of Nano-Se improved the body weight, feed consumption ratio, antioxidant status, immunity and tissue Se deposition in grower birds.