Abstract: Flow-induced vibration of two circular cylinders arranged in tandem at Re=100 is numerically investigated. The mass ratio of the cylinders is 2.0 and the center to center spacing ratio of the cylinders varies from 2.0 to 5.0. Two scenarios are considered: (a) the downstream cylinder is allowed to vibrate freely in the cross-flow direction while the upstream is fixed; (b) both cylinders are allowed to vibrate freely in the cross-flow direction. Results show that no matter the upstream cylinder is fixed or not, the transverse vibration amplitude of the downstream cylinder is obviously larger than an isolated one. For Scenario (a), the transverse amplitude of the downstream cylinder is larger than that of Scenario (b), which can be attributed to the fact below. When both cylinders vibrate freely, a significant "mutual-adjusting" happens between the wake of the upstream cylinder and the vibration of the downstream one, which intensifies the interaction between the two cylinders. We also investigated the coupling mechanisms of the downstream cylinder vibration and the gap flow of Scenario (b) and found that the vibration amplitude of the downstream cylinder attains its maximum when the reattached shear layer from the upstream cylinder can completely pass through the gap.