Fourier phase differences of earthquake motions are investigated by deriving the relationship between the phase differences and the envelope function of narrow band components containing a certain frequency of the Fourier spectra. By making use of the relationship, it is verified that the probability distribution of phase differences is closely related to the envelope of earthquake motion. The relationship is also used to analyze the nonstationary frequency content of recorded accelerograms, the result of which is compared with evolutionary spectra. It is concluded that the phase differences are useful to evaluate the nonstationary frequency content of earthquake motions. It is suggested that the concept is also useful for the simulation of earthquake motions.