The Dynamic behavior of the JT-60 vacuum vessel was examined during its power tests to confirm the validity of the dynamic response analyses of the vacuum vessel. Observed accelerations and displacements showed good linearity with electromagnetic forces and reached maxima of 48 m/s2 in acceleration and 0.46 mm in displacement at full power operation. The observed natural frequency was approximately 50 Hz, showing good agreement with that predicted by the dynamic response analysis. Dynamic response analyses showed that the vibration mode with a natural frequency of 46.3 Hz had a displacement amplitude of around 0.1 mm at the rigid ring under the misfire start-up operation conditions. The difference between the displacement predicted in the dynamic response analyses and that observed at the lateral port could be explained by the lever ratio effect of the lateral port.