Effect of green tea extract, on duodenal ulceration was studied in male Sprague Dawley rats treated with cysteamine, a drug, which causes duodenal ulcers in experimental animal. As a result, in the proximal duodenum, a significant decrease of ulceration was detected twenty four hours after cysteamine injection in rats raised in green tea extract for 63days. Special reference to duodenal alkaline phosphatase activity was measured in mucosal homogenates. In control rats raised in tap water Riven saline, significant decrease was observed in proximal duodenal alkaline phosphataes activity. The decrease effect seems site specific, since the enzyme in the distal duodenum remains. Moreover the effect cysteamine in control rats alkaline phosphatase is specific, because, in rats raised in green tea extracts did not show significant change in activity. It is suggested that green tea extract acts in ideal properties as an anti-duodenal ulcer agent.