I studied histologically the temporal bone preparations (40 ears in 30 cases) from humans ranging wide in age from newborns to elderly porsons, with predominance on those from younger children. Morphological changes with age in facial nerve and facial canal were studied and compared with those in adults. The facial nerve in newborns has a diameter of about two thirds of that of adults, achieving a growth of about 1.6 folds after birth in terms of diameter. On the other hand, since the facial canal has a diameter corresponding to three fourths of that of adults already at birth, its growth rate is somewhat lower (about 1.4 folds). The wall of the facial canal on the side of the tympanic cavity is 0.12 to 0.24 in thickness and the wall of the facial canal on the side of mastoid cavity about 1 mm on average. The wall on the side of the tympanic cavity is somewhat thicker in newborns than in adults, decreasing with aging. At age of about 5 years, it grows to the thickness almost corresponding to that of adults. The unmatured wall of the facial canal on the side of the mastoid cavity also increases its thickness till the age of about 3 years, followed by a slight tendency of decreased in thickness. These changes in thickness of the osseous wall may be attributable to bone resorption of air cell system which is in contact with it. In the wall on the side of the tympanic cavity adjacent to the oval window, apparent bone defect was observed with very high incidences in both children and adults. Therefore not only during the operation of the middle ear but also in ordinary clinical practice the presence of bone defect in this area should be taken into consideration. It is considered that dehiscence appears due to incomplete fusion of the bone wall during the facial canal development process in the embryonal stage. In addition, it may be formed after birth owing to pneumatization associated with growth of the middle ear.