Abstract: Rotor angle instability and transient voltage instability are two forms of system instability during the post disturbance and may occur at the same time. Recognition between them is the pre-preparation for power system emergency control. This paper analyzed the characteristics of the active power on tie lines in large scale connected power systems and extracted the principal system variables reflecting the system unstable modes based on total differential equation of active power. Then the relationship between different unstable modes and principal system variables was shown and a method for recognizing principal modes was presented. This method can take full consideration of the system dynamic characteristic, and only needs real time measurements to calculate. Moreover, the method has clear physical meaning, is simple to use and has fast calculation speed. Simulation results show that the method is effective.