Vortex-induced vibrations (VIV) can cause fatigue damage of marine pipeline and mooring lines.The current VIV studies focus on a canonical problem that a circular cylinder freely to oscillates within a flow perpendicular to the body axis.In practical applications,cylinder structures are often inclined with respect to the direction of the oncoming flow.Aiming at this complex situation,the independence principle (IP) that the VIV of an inclined circular cylinder is assumed to be equivalent to the VIV of a vertical circular cylinder excited by the projection component of oncoming flow velocity in the direction perpendicnlar to the cylinder’s axis is proposed.However,more attentions need to be paid to the validity of the independence principle.Here,laboratory tests were conducted for vortex-induced vibrations (VIV) of an inclined flexible cylinder with a ratio of length to diameter of 350 and an incline angle of 45° in order to check the validity of the independence principle (IP).Strain gages were adopted to measure the cylinder’s dynamic response,a modal analysis approach was used to analyze the measured data.The test results of a vertical cylinder and the inclined cylinder were compared.It was shown that the higher order modes are easy to be excited for the inclined cylinder in the controlled mode transition region; the strain and displacement at measured points of both cylinders are obviously different and the higher order modes’ features of VIV responses are observed.All the results indicated that the use of IP may not be correct to describe the VIV of the flexible cylinder inclined at 45°.