A method to simplify strong ground motions to 1 cycle sine waves was investigated by taking into account inelastic seismic responses of structures. Elastic and inelastic earthquake response analyses and response analyses under the input of sine waves were made with single-degree-of-freedom-systems. Strong ground motions could be simplified to 1 cycle sine waves if the inelastic responses of structures were considered. A method to simplify strong ground motion to a 1 cycle sine wave whose acceleration amplitude is the peak ground acceleration and whose period is the effective period which is defined in Eq.(3) in this paper is proposed.