The present study was conducted on 180 male chicks (RMBCs) that categorized into nine groups based on the post hatching age: D1, D4, D7, D14, D21, D28, D35, D42 and D58. To accomplish this research, the routine and special stains as well as mouse anti-chicken; monoclonal anti-Bu-1 antibody marker were employed. The epithelium of the bursa of Fabricius possessed simple columnar epithelium, M-cells and intraepithelial lymphocytes. The M-cells were characterized by vesicular nuclei and pale cytoplasm that did not stain with H&E, PAS and Diastase-PAS. The lymphoid follicles of the bursa consisted mainly of lymphocytes as well as lymphoblasts, reticular cells, plasma cells, macrophages and dendritic cells. The bursal lymphoid follicles obviously appeared at D1 old and then continued during bursal age. At D7 old, layer of cuboidal cells associated with blood capillaries clearly appeared in corticomedullary border. The number of the lymphoid follicles was gradually decreased significantly (P < 0 .05) with the advance age, whereas the diameter of the same follicles increased significantly (P < 0 .05) with progress of age also. The Bu-1 positive B-lymphocytes were chiefly located in cortex and medulla of the lymphoid follicles from D1-D58 old. The study concluded that the maturation of the lymphoid follicles of the bursa in this breed was depending on age