Abstarct: Growing demand for electricity consumption and competitive electricity markets has led to fast changes in operation of power systems. Consequently, the transmission lines overloaded. As a result, the generating units of power systems are highly faced to instability. This kind of instability mainly happens when grid is highly overloaded and some several outages happen in a short period of time. These outages cause power swing in generating units and nearby grids. Therefore, due to excessive voltage drop and lose of synchronism, the instability is happened. The transmission lines are protected by distance relays. One of the main reasons of cascading outages after a large disturbance is wrong operation of distance relays. To prevent these unwanted events, the wrong operation of distance relay in power swing mode must be avoided. In this thesis, the main reason of power swing is discussed and system behavior during this mode is analyzed, then several conventional techniques in power swing are reviewed briefly. Since, the wavelet transform is used in this thesis to analyze the power swing phenomenon, this transform is reviewed briefly. After that, the proposed algorithm which uses energy of first to fourth levels of wavelet transform of voltage waveform for power swing detection is employed and then, simulation results are presented. Finally, in the last chapter conclusion and results are presented.