The normal round window membrane in human fetus was examined by transmission electron microscopy. The membranes in 5 to 9 months old fetus consist of the following three layers: (1) an outer squamous epithelial layer facing the middle ear, there was no epithelium in 4-month-old stage, (2) a middle fibrous layer consists of collagenous and elastic fibrils, fibroblasts, fibrocytes, blood vessels, and nerves, (3) an inner layer of simple flat cell(mesothelial cell) facing the scala tympani. With the advance of age the epithelium is getting thinner, the number of fibroblasts is reduced but that of fibrocytes is increased, collagenous and elastic fibrils grow gradually in density.The round window membrane in 8-month-old fetus is simillar to the ultrastrucrural characteristics of that in adult. The round window membrane has an important and complicated physiological function. The epithelium of the tympanic layer with tight junction and many desmosomes appear to present a barrier to free diffusion through the intact membrane. The stability and elasticity of the membrane provide protection against rupture and elasticity plays a role in the physiology of hearing as well.