The out-of-stability state of synchronous generator which connected to high voltage network due to the transient situations which lead us to disconnect the generator causes finincial and technical problems. Therfore the purpose of this research is to study the stability of synchronous generator working which connected to high voltage network through the transient situation.This research present the posibilty of returning the synchronous generator to synchronous-steadystate at the late time of the transient state when the load angle is in the out-of-stability zone through controling the rotor-excitation and increasing the generator’s electric motive force.This lead to increase the generator’s maximum power and to increase the generator’s stability-torque which lead to return the synchronous generator to synchronous-steadystate without need of discconecting it from network.Then put the mathmatical model.