Abstract: In the actual system, the ultra-low-frequency frequency oscillation（ ULFFO） event caused by instability of the primary frequency regulation process is different from the traditional low-frequency oscillation in both mechanism and performance. ULFFO belongs in the category of frequency stability. The single-machine single-load system is the simplest system to study this problem. Such key characteristics of ULFFO as oscillation frequency, damping and oscillation performance are tackled based on the single-machine single-load system. The oscillation frequency and damping as well as their influencing factors are derived and analyzed in the simplified model. The Bode plot method, whose crossover frequency and phase margin corresponds to the oscillation frequency and damping ratio respectively, is used to investigate the oscillation frequency and damping in the detailed model. The applicability of the damping torque method in treating the damping of the prime mover system is confirmed. The amplitude of mechanical power is larger than that of electromagnetic power in ULFFO.