Abstract: The vortex-induced vibration of a flexible slender riser in cross-flow with a low Reynolds number was numerically investigated. The methodology adopted was a combination of CgLES_IBM – an in-house 3D parallel CFD code based on the immersed boundary method (IBM) and X-code – an in-house implicit structural dynamic code. It was found that the vortex-induced vibration (VIV) responses show clear standing-wave characteristics. The in-line vibration is dominated by odd-number modes while the odd-even property of the cross-flow vibration is determined by which natural frequency is close to the vibration frequency. The power spectral density (PSD) of VIV shows a single-spectrum mode – all vibrating energy concentrates in a narrow frequency band. Lower PSD is observed at the node, while higher PSD anti-node. At the node, time-averaged energy transfer coefficient C_fv is negative which means that energy is transferred from vibration system to outer flow, and thus VIV is suppressed. Between two subsequent nodes, C_fv is positive and shows a saddle-shaped curve along the span-wise direction. Energy is transferred in the opposite direction and VIV is excited. Obvious three-dimensionality is detected in the downstream near-wake in which the 2S mode dominated. Due to the span-wise transporting of the vortices caused by oblique vortex-shedding, the near-wake pattern changes to a 2P₀ mode. However, the near-wake pattern is dominated by the 2S mode again when the weaker vortex in the 2P₀ mode dissipates.