There are many factors contributing to the etiology of otitis media with effusion, and none of them is actually proven to be true. The mechanism of fluid accumulation in the middle ear is thought to be caused by Eustachian tube obstruction or impairments in the mucociliary function of the tubotympanic. The purpose of this study was to investigate the morphologic changes of the middle ear mucosa in 10 children with otitis media with effusion using the trasmission electron microscopy. Middle ear mucosa was taken as the biopsy specimen during the ventilation tube insertion. We observed only a few number of ciliary abnormalities, but a significant decrement in the number of ciliated cells with increased secretory and increased thickness of mucous layer within the mucous blanket was identified. Therefore, a decreased number of ciliated cells and increased thickness of the mucous layer may cause mucociliary dysfunction in otitis media with effusion.