Abstract: To investigate the phase changing processes intraffic flow, we simulated a single lane traffic flow using a cellularautomation. By extracting the section flux data and using discrete Fouriertransform method, we made the power spectrum analysis. From the result wefoundthat in the phase of jam, there exists a same period as that of atraffic shockwave resulting from our using the periodic boundaryconditions; in the phase of heavy synchronized flow with high roadoccupancy, there still exists some periodic waves which are verydifferentfrom the traffic shockwave, and we could not attribute them to theperiodic boundary conditions; in the phase of free flow under low roadoccupancy, no periodic waves exist.