This study was performed to investigate the influcence of mugwort on the duodenal ulcer induced by cysteamine administration in rats. Five groups of rats were fed each experimental diet containing 0%, 5%, 15%, 30% of mugwork powder for 10 weeks. Duodenal ulcer was induced by cysteamine injection (400mg/100g body weight) after 10 weeks of feeding experimental diets (C-0, C-5, C-15, C-30). Control animal that fed 0% mugwork powder added diet were injected saline (S-0) to compare with cysteamine injected groups. When the duodenal ulcer induced by cysteamine-HCI administration, all animals in the C-0 group formed erosion and perforating ulcer was found in 44% of animals. Higher the added mugwork ratio, more inhibited of the duodenal ulcer induced by cysteamine administration (C-5, C-15). But when the ratio of added mugwort is 30%, the inhibition effect disappeared (C-30). The alkaline phospatase activities were lower at the duodenal mucosa and small intestinal mucosa in the cysteamine treated groups(p<0.05). The acid phophatase activities were higher at the stomach, small intestine and large intestine of the cysteamine treated groups. But in mugwort added diet group, the changes of enzyme activites were lessended. The maltast activities were lower at the duodental mucosa and small intestinal mucosa of cysteamine treated groups. But in mugwort added diet group, maltase activites were recovered.