Modeling phase characteristics of earthquake ground motion is important to synthesize a design earthquake motion consistent with given response spectra. This paper presents two simple methods to model phase characteristics of earthquake motion using the concept of group delay time. One is a theoretical method to model the group delay time of a train of impulses which represent fault ruptures. The phase shift due to the transmitting path of seismic wave motion is also expressed by the minimum phase transfer function of its amplitude spectrum. The other one is an empirical method to model the group delay time based on observed records. The wavelet transformation is applied to extracting non-stationary characteristics of group delay time of earthquake motion in the time-frequency domain. Then, the average and standard deviation of group delay time of each earthquake motion are calculated on each compact support section of wavelet transformation.