This study evaluated the relationship between occlusal force and tooth displacement in function. The three-dimensional tooth displacement transducer Type M-3 was used to measure the tooth displacement path and a three-dimensional small occlusal force sensor was developed to detect the occlusal force. Subjects' teeth were the upper and lower first molars. The three-dimensional tooth displacement and occlusal force of the upper and lower first molars were measured at the same time during light and heavy clenching in vivo. The direction of the tooth displacement and that of the occlusal force of the upper first molar almost coincided and they were along the palato-apical direction. However, in the lower first molar, the direction of occlusal force was in the apical direction, and the tooth displacement path was along the lingual direction. There was no relationship between the direction of occlusal force and clenching intensity at the upper and lower first molars and no relationship was observed between the direction of tooth displacement and occlusal force at the lower first molar. However, the upper molars showed a tendency to move more in the palatal direction as the force increased. In the case of the upper first molar, the energy of force seemed to change with the distortion of the alveolar bone especially around the root surface. However, in the case of the lower first molar, the energy of this occlusal force might be absorbed by the deformation of the mandible.