The essential feature of this study is harmonic resonance of floating compliant structures which are elastically vibrating in regular waves. The conceps of “characteristic bending moment” which is determined with no consideration about structural elasticity is introduced. It is proved to be efficient in estimating significant wave length condition ( L /λ) for various vibration mode shape on which significant bending moment response appears. The bending moment response is amplified by flexural vibration. The amplification factor of resonant bending moment is dependent primarily on the effectiveness factor of the flexural rigidity of the structure which is defined as EI ( m_j / L ⁴)/ k . Consequently, the amplitude of harmonically resonant bending moment is found to be estimated through the characteristic bending moment and the amplification factor. Significant third harmonic resonance which is induced by non-linear wave force was observed in the experiment. Super-harmonic resonance in bending moment response is investigated through numerical calculation using higher order term of drag force which is expressed in Morison's equation.