Tube bundles in cross-#ow vibrate in response to motion-induced #uid-dynamic forces; hence, the resultant motions are considered to be a #uidelastic vibration. The characteristics of the vibration depend greatly on the features of the #uid-dynamic forces and the structure of the tube bundle. Therefore, in this study, the equations of motion of the tube bundle are derived. From the viewpoint of vibration, each tube is not independent of the surrounding tubes because its vibration is a!ected by #uid-dynamic coupling with the neighboring tubes. Thus, the equations are a set of coupled equations and the solution is obtained as an eigenvalue problem. The #uid-dynamic forces, which are indispensable in the calculation, have been obtained by experiments using a vibrating tube in the bundle; it was found that the forces depend strongly on the reduced velocity. Using these equations and the #uid forces, critical velocities of the tube bundle vibration are calculated, and it is found that the critical velocity is strongly dependent on the #uid-dynamic force characteristics, as they vary with the reduced velocity. Vibration tests of the tube bundle have also been conducted, and the critical velocities obtained in the tests are compared with the calculated values; agreement with the calculated values is good, demonstrating that the method of calculation is useful. The e!ects of mass ratio, frequency deviation and damping deviation of tubes in the bundle on the critical velocity are also examined theoretically. It is found that it is better to treat the mass ratio and the logarithmic decrement separately when the mass ratio is less than 10. Di!erences in natural frequencies make the critical velocity large. Similarly, di!erences in logarithmic decrement may distribute the vibration energy to other tubes and make the critical velocity large.