Abstract: In-line and cross-flow vortex-induced vibration (VIV) offlexible cylinders is studied based on the VIV experiment. The relationshipof two-degree-of-freedom of vortex-induced vibration of flexible cylindersis also investigated. A nonlinear model, with fluid-structure interaction,of two-degree-of-freedom VIV of flexible cylinders is then proposed. It isshown that the ratio of the frequencies and amplitudes of in-line and crossflow VIV of the cylinders changes with current velocity/Reynolds number. Theratio of the VIV frequencies is 1.0 beyond the lock in district and 2.0within the lock in district, respectively. And the ratio of the VIVamplitudes is 1.0 beyond the lock in district and 1/3 to 2/3 within the lockin district.