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As the power system is interconnected with HVDC transmission, the power oscillation with low frequency appears in the AC transmission lines with weak damping. Due to the existence of the low frequency oscillation (LFO), the transmission power of AC lines is limited and the system angle stability is affected. In this paper, the low frequency oscillation (LFO) phenomena are introduced. By using the eigenvalue analysis method of small signal stability, the frequency and oscillation model of LFO in a two-area, four-machine model with AC and HVDC in parallel are analyzed. In the system model, the generator is identified as a three-order reduced-model, and HVDC is described as a two-order model, then the mathematical model of the whole system is expressed as linearized state equations. By calculating the eigenvalue of the linearized matrix, the system oscillation modes with damping ratio are analyzed in this paper. By using the phase signals of PMU's output, the corresponding supplementary controllers for HVDC hybrid transmission are discussed simply for the future work.