Effects of different feeding levels on organ development and tissue morphology of reticulum and omasum of Holstein bulls at the age of 6 months were studied in this paper. Nine healthy Holstein bulls were used and divided into three groups randomly. Dietary concentrate to forage ratio were 6∶4 (groupⅠ), 5∶5 (groupⅡ) and 4∶6 (group Ⅲ), respectively. Organ development and tissue morphology of reticulum and omasum of three groups were investigated by tissue section making and electron microscopic observation after 3 months. The results showed that: 1)mucosal structure of reticulum and omasum in three groups were all integrated; 2)fresh weight and the ratio of fresh weight to body weight of reticulum and omasum raised with the increase of the proportion of forage. Fresh weight of reticulum and omasum in group Ⅲ were 21.90% and 19.93% higher than that in group Ⅰ(P＜0.05) and ratio of fresh weight to body weight in group Ⅲ were 29.86 % and 27.68 % higher than that in group Ⅰ(P＜0.05); 3)the development of reticulum and omasum was better with the increase contents of forage. Compared with other groups, thickness of stratum corneum, width of lamina propria and apparent development of nipple in group Ⅲ were significantly increased (P＜0.05). In conclusion, forage in diet could improve the development of reticulum and omasum organ and the change of tissue morphology of Holstein bulls.［Chinese Journal of Animal Nutrition, ２００9，２1（5）:792-797］