In the new century, the use of most growth promoter antibiotics has been forbidden by the European Union due to their side effects such as antibiotic residues in poultry products and development of antibiotic resistance. The medicinal plants and their extracts have received increasing attention as possible alternatives for antibiotics [1]. Oregano with the scientific name Mentha longifolia is a medicinal plant from Laminaceae family which is used for medical and culinary purposes [2]. The major components of oregano essence are terpenoid compounds such as thymol and carvacrol [3] which have antimicrobial and antioxidant properties [2]. It has been reported that the beneficial effects of essential oils should not only arise from their antimicrobial properties but also from their contribution in immune system function [3]. In the present study, 480 one-day old Ross 308 broiler chicken were randomly allotted to 6 treatments, 5 replicates and 16 birds per each replication. The experimental treatments consisted of control (with no feed additive), 0.02% Virginiamycin as growth promoter antibiotic, and 0.25%, 0.50%, 0.75% and 1.0% oregano powder. At 16 day of age, Newcastle disease vaccine was administered orally to all chicks and 6 days thereafter, blood samples were taken from two birds of each cage via wing vein for determination of antibody titer against Newcastle vaccine. At the end of experimental period (42 day of age), two birds from each cage, close to the cage average weight were selected and slaughtered after recording their live body weights and their thymus, spleen and bursa of Fabricius gland weights were measured in proportion to live body weight. The data were analyzed in a completely randomized design using the GLM procedure of SAS. Comparison of means was conducted by Duncan's multiple range test. The results of current study indicated that the experimental treatments had no significant effect on thymus, spleen and bursa of Fabricius percentage. It has be
