The large amplitude forced power oscillation on the tie line may be caused by the sustained periodical small amplitude disturbance due to the resonance. However， it is difficult to detect or capture the disturbance source. Two kinds of oscillation， which are caused by the prime mover power disturbance and the load disturbance， are compared theoretically on a two-machine equivalent system. Furthermore， for these two kinds of disturbance source， the different properties are discussed; and time-domain simulation of forced power oscillation is carried out by MATLAB. The results show that the amplitude of forced power oscillation caused by the prime mover power disturbance is even larger than that of load disturbance in the case of the same disturbance amplitude and frequency. This conclusion can provide reference to understand low frequency oscillation phenomenon occurred presently in power systems.