Modeling the phase characteristics of earthquake ground motion is important in synthesizing a design earthquake motion consistent with a given set of response spectra. We assume that earthquake ground motion can be expressed by a convolution of the three time functions of source, path and site effect. This paper presents a new methodology to model the phase characteristics of earthquake motion using the concept of group delay time. The group delay times of source effects caused by rupture propagation on the fault plane are theoretically calculated, while those of the path and site effects are empirically modeled from observed records using the inversion technique. We also demonstrate that earthquake motion can be synthesized based on our newly developed phase model.