Abstract: In order to avoid an arbitrary selection of two reference vibration modes which are used to evaluate the two coefficients of Rayleigh damping,an optimization method for direct determination of the two Rayleigh damping coefficients is proposed. In the method,by formulating and minimizing an objective function which is an error term of the peak response at certain degree of a structure,the two coefficients can be systematically evaluated following the deformation response spectrum of excitation and modal analysis. Then,based on the harmonic response analysis of a five-storey shear building,the effects of both spatial distribution and frequency content of excitations and the effects of structural dynamic characteristics on selection of natural frequencies in Rayleigh damping are investigated in detail,and the accuracy and effectiveness of the proposed method are demonstrated.