Characteristics of the voltage frequency and current frequency of oscillation center and non-oscillation-center are analyzed. Studies show that during power system oscillations， the voltage frequency is equal to the current frequency only in oscillation center. On one side of the oscillation center， the voltage frequency is greater than the current frequency and on the other side， the voltage frequency is lower than the current frequency. Frequency differences between voltage frequency and current frequency depend on the oscillation period， electrical distance between measurement point and out-of-step unit and electromotive force angle of out-of-step units. A new oscillation identification method is proposed based on the frequency differences between electrical quantities. Wide-area measurements can be used to determine the location of oscillation center. Finally， the simulation results verify the correctness of the analysis.