Objective: On a cervical disk, the bulge may be produced in the poster lateral direction when there is a combination of lateral bending and compression loading. Although attempts have been made to measure the kinematics of intact vertebral spine but has not been compared with a vertebral spine when the dicks are bulged. The study aims to investigate the effect of a bulged disk on the kinematics of a cervical spine during a flexionextension motion then flexion-extension of five subject patients who had history of bulged disk in their cervical spine were studied. All patients had bulged disk at C5/C6 level. Design: This study uses a three dimensional model of cervical vertebras which was created from CT data of the subject patients. Flexion-extension range of motion of each subject patients was recorded using a fluoroscopic imaging method. Then motion of the cervical vertebra next to bulged disk were tracked using a 2D-to-3D registration method. The result of this measurement was compared with motion of intact cervical spines obtained by previous researchers. Results: The range of motion for normal C5/C6 is less than bulged C5/C6. Conclusion: The kinematics of flexion-extension motion for bulged intervertebral disk and the normal intervertebral disks are different.