An influence of the voltage sag on the shaft torque of the nonutility generator connecting to the utility system is investigated. During the oscillation of the generator electromagnetic torque, the mechanical shaft torque also oscillates according to the mechanical oscillation modes. In this situation, the shaft might be damaged. Three phase voltage sags are simulated by using ATP-EMTP, changing the fault point impedance and the short-circuit impedance of the utility system while the transformer impedance and the generator parameters are kept constant. The state equation of the generator mechanical system is investigated by using MATLAB/SIMULINK. Numerical simulation results show that the waveform of the electromagnetic torque is influenced not only by the fault point impedance but also by the short circuit impedance of the utility system, and thus a special condition of both impedances, in which the shaft torque has a maximum amplitude, could be found.