Serotonin (5-HT) exists in all kinds of animals' digestive tract, which has some important physiology effects on the growth and reproduction of crustaceans. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme for 5-HT biosynthesis and can be used to identify 5-HT as a marker. The morphology and distribution of TPH in the midgut, the hindgut ampulla and hindgut of Eriocheir sinensis were mainly studied in this research by using histology and immunohistochemical technique. The result shows that: the histology of the crab intestine in midgut is similar to that in hindgut. There four layers can be identified in midgut and hindgut including the mucosa, submucosa, muscularis and adventitia. However, the size of adventitia of the hindgut ampulla composed of connective tissue is larger than that of midgut and hindgut. Except adventitia, the histology of distant rostrol and near rostrol of hindgut ampulla is similar to midgut and hindgnut respectively. TPH can be detected by immunolocalization in the midgut, the hindgut ampulla and hindgut. In the midgut, TPH is mainly distributed in the nucleus of mucosal epithelial cells. In the hindgut ampulla, TPH was widely distributed in the nucleus of mucosal epithelial cells close to the midgut and the cytoplasm of mucosal epithelial cells close to the hindgut,and peripheral connective tissue. In the hindgut, TPH is mainly found in the cytoplasm of mucosal epithelial cells. These results will be helpful for future studies on physiological regulation mechanism of 5-HT in the digestive tract of the crustaceans.