To observe the ultrastructure of mucous membrane of Eustachian tube in human being, ninteen human's Eustachian tube from various ages, were divided into tympanic segment, isthmus and pharyngeal segment and observed under SEM. The results showed that the epithelium of Eustachian tube consisted of ciliated cells, nonciliated cells and goblet cells. At the isthmus, the bone and cartilage junction, the cells got crowded, and there were more and more goblet cells from tympanic side to pharyngeal side, as well as the cilia of ciliated cell leaned towards the pharyngeal side. The upper one third part of Eustachian tube was covered mostly by nonciliated cells, the floor and lower parts were ciliated cells. It was concluded that the upper parts of Eustachian tube's wall played an important role in maintaining equation of pressure in middle ear, and the floor and lower parts in providing an active mucociliary mechnism. The defence system of the Eustachian tube mucous membrane had developed during the fetal stage and was well established at birth.