Abstract: Instantaneous frequency should be used to reflect power system characterization in the process of out-of-step oscillation. A two-machine oscillator model based on instantaneous frequency was set up to study voltage and current frequency response in the process of out-of-step oscillation. Voltage instantaneous frequency of out-of-step oscillation center is equal to the unit frequency combined. The bus voltage instantaneous frequency move in opposite direction and have no intersection in two sides of the out of step oscillation center. Voltage instantaneous frequency reaches its extreme value when power system angle is 180°. The current instantaneous frequency between the two unit frequency if the oscillation center falls within the system, and only when both units have the same voltage amplitude, the instantaneous frequency is equal to the instantaneous voltage of out of step oscillation center. A method to pinpoint oscillation center based on instantaneous frequency was put forward. Oscillation splitting device installed in substation can not only judge that whether the oscillation center is on line but also can accurately pinpoint the oscillation center position, by simply introducing voltage instantaneous frequency information of the other substation. Simulation results verify the correctness of oscillation center location method.