Abstract: The response of SDOF system to harmonic excitation is a classical topic in structural dynamics,it is also the basic theory used in earthquake engineering.Therefore,the study of response spectra to harmonic ground motion can provide some insight in the characteristics of real earthquake ground motion,particularly those generated by near-source earthquake.Based on the theory of structural dynamics,the complete solution,steady state solution and transient solution as well as their respective response spectra of SDOF systems subjected to harmonic ground motions are derived and the relations among these three kinds of response spectra are discussed.It is concluded that the transient response to a high frequency harmonic ground motion can increase greatly the displacement response of structures,particularly for the long period structures.The transient vibration should not be ignored in the earthquake-resistant design of real structures that are close to the causative faults.