It is well-known that a cylinder bundle vibrates in a cross flow. Studies of the vibration have been made by Roberts, Connors, Blevins et aL, and it has been established that the vibration is a fluid elastic vibration. However, their theory, which is based on quasi-static fluid forces, does not always hold good for all vibration phenomena. In the theory used in this paper unsteady fluid dynamic forces are considered, which are induced by the vibrating cylinders. Since theoretical prediction of unsteady fluid dynamic forces is difficult, model tests were conducted to measure the fluid,forces. The eqhations of motion of the cylinders were deduced and critical velocities were calculated by using the measured unsteady fluid dynamic forces. Critical velocity tests were also conducted with cylinders supported by elastic spars. The calculated critical velocities coincided well with the test results. Effects of fluid density on the critical velocity were studied and it was found'that the critical velocity in a low density fluid like air is proportional to the one half power of the mass damping parameter, as predicted by the previous theory. However, the critical velocity in a high density fluid is less influenced by the mass damping parameter. The effects of detuning of the natural frequency on the critical velocity were also cor~sidered. 9 I..-