Tooth morphology has an important role in esthetical characteristics, functional performance and physiological action. Mandibular second premolars among teeth were taken for a close investigation with a micro-CT for a non-destructive measurement. Thus, the shape of the tooth was classified into a few partitions and measured by setting the reference axis to express internal and external morphology numerically. About a thousand tomographic images were obtained for each specimen and were reconstructed to a three-dimensional form. The lengths and widths of each dental tissue were measured from the three-dimensional form. In this tooth type, the width of dentine is comparatively larger than that of enamel, and buccal tissue of the crown is thicker than lingual tissue. Hence, buccal tissue could afford to withstand against exterior impact or stimulus. Finite element models were drawn up, applied with an ordinary occlusal force and analyzed to get the stress distribution. Thus it was shown that the irregular feature of the tooth is not useful only to masticating and pronouncing as well known, but that it is also suitable for protecting inner tissue by dispersing stress and delivering proper pressure to periodontal tissue to continue a physiological action. These results could be used for development of dental implants and clinical trials.