This work presents a method to determine rotor oscillations during a three-phased sudden short-circuit test of a synchronous generator. To our best current knowledge, no such similar method is yet published in the technical literature. During the periodic and non-periodic component separation of the three phase currents, it was found that the time difference between the currents' extreme values, for any of the three phase currents, does not have a 10 ms constant value, as in the stationary phase. Investigating why this happens we found that these variations are caused by the rotor oscillations. Certain parameters of a synchronous generator's rotor and of the rotor's oscillations are measured during the sudden three-phased short-circuit test. Based on these readings, with the method presented in this paper, we can compute the rotor's mechanical oscillation amplitudes and the transient process durations. Using the measured values, the equation describing the rotor's mechanical oscillations, independent of its power, can be re-written. Knowledge of the oscillation amplitudes and their durations is necessary for a correct loop control design and in establishing the generator's performance.