Abstract: The nonstationary characteristic of earthquake ground motion has been clearly understood, however, its effect on the structural nonlinear responses has not been studied comprehensively. The traditional response spectrum and power spectrum do not have the time information of earthquake ground motion, whereas the local spectra (time dependant spectra) by wavelet transform can reflect the nonstationary characteristic more precisely. Especially, wavelet transform has the ability of redistributing the energy of earthquake ground motion in the time domain without any changes in frequency, which is advantageous to the analysis of the effect of nonstationary characteristic on structural responses. A comparison of the effects of three earthquake records, and two altered versions of one of the records with the energy being redistributed by wavelet transform, is carried out. Based on the comparison, the authors point out that the ernergy concentration of earthquake ground motion in the time domain, which is also one kind of nonstationary characteristic, is especially harmful to a structure. Consequently a notion of effective peak is proposed to quantitatively represent the energy concentration.