The fine structure of Sharpey's fibers in the alveolar bone of rat molars were observed by electron microscope. The fixed samples were three-dimensionally observed with a scanning electron microscope (SEM) after NaOH treatment. The penetrating Sharpey's fibers in the alveolar bone appeared distinctly. The fibers penetrated straight and/or branched in the alveolar bone. The Sharpey's fiber bundles ran nearly perpendicular to the bone matrix fiber bundles. The other fixed samples were embedded in polyester resin, ground and observed by backscattered electron signals in an SEM (BSE). In the BSE images, even thin Sharpey's fibers deeply penetrating the alveolar bone were hypomineralized. The fine ultrastructure of fibers were also observed by a transmission electron microscope (TEM). It was suggested that the three-dimensional structure of Sharpey's fibers intensified the continuity between the periodontal ligament fiber and the alveolar bone and consisted of a buffer medium against stress.