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Fluid added mass/damping have significant influence on the vortex-induced vibration responses of low mass-damping ratio circular cylinder.Added damping coefficient which concerned added mass and structural "real" natural frequency in fluid was defined by referring to structural damping ratio.Added mass/damping ratios were calculated through two methods.One was to assume the harmonic response of the oscillation system; the other was to relate them with the harmonic response in the dynamic equilibrium equation,then time sequences of response which were calculated numerically were utilized.It is found that the results came from the two procedures were almost the same:added mass (coefficient) was basically decreasing during vortex-induced resonance; in contrast,added damping and its coefficient were increasing in this region,but the latter changed its sign near the middle of the resonance region.In addition,the "real" natural frequency collapses well with the vibration frequency during resonance.