Summary This study was conducted because of influence of prebiotics (alphamune) and symbiotic (biomin) on histometrical development of small intestine and lymphatic tissue of caecal tonsil have been not reported on broiler chicken. In this study, different parts of small intestine and caecal tonsil, which is a major area of gut associated lymphatic tissue (GALT) in chicken were selected. One hundred and twenty-five 1-day-old broiler chicks were divided into 3 groups: 1) Control group which were fed only by basal diet, 2) Alphamune group which were fed basal diet supplemented with prebiotic (0.5 kg/ton alphamune), and 3) Biomin group, fed basal diet supplemented with symbiotic (1 kg/ton biomin). At the end of 42 days, 15 chicks (5 chicks from each group) were selected for histomorphometrical study. The samples were taken from different parts of small intestine (duodenum, jejunum and ileum) and caecal tonsils and fixed in 10% formalin. The 5-6 µ sections were made by paraffin embedding method and stained by H&E. The results showed that although alphamune and biomin have a significant improvement on histomorphological structure of small intestine (more significant changes were seen in duodenum) and lymphatic tissue of caecal tonsil, the biomin has a more significant improvement. Villous height and thickness, the number of goblet cells, number and depth of intestinal crypt, density and area of aggregated lymphatic tissue were increased more significantly in biomin group (P<0.05). The knowledge obtained from this study showed that alphamune and biomin are considerable feed additives instead of antibiotic for improvement of histomorphological structure of small intestine and its associate lymphatic tissue.