Tooth displacement due to occlusal force was measured using X-ray CT images and coordinate transformation method. The two CT images of mandibular dentition were recorded without and with the occlusal force. The tooth displacement was evaluated from the two images by matching them at the mandibular bone region using the coordinate transformation method. The following results were obtained under the occlusal force of about 100N at the first premolar: (1) The displacement measurement of the tooth is possible using the present method. (2) The displacement was also observed at the teeth without occlusal force. (3) The displacement tended to be small in the direction of tooth axis but relatively large on the occlusal surface. (4) The absolute values of the displacement were less than several hundred micrometers in the tooth axis and were nearly one millimeter on the occlusal surface.