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The increase of the large scale wind power integration have affected many aspects of power systems operation and behaviors. This paper focuses on the impact of a grid-connected direct-drive permanent magnet generator (DDPMG) wind turbine system on damping of power systems oscillations. In this paper, a detailed model of a DDPMG with controllers which is suitable for the small signal stability analysis is presented. The state-space equations of a two-machine system have been derived to illustrate the mode decouple characteristic between a DDPMG wind system and power system and the insight meaning of this decouple concept has been clarified by theoretical analysis. The modal analysis technique has been adopted to reveal the inherent damping characteristics of a DDPMG wind turbine system and the impact of system parameters on stability, thereby to confirm the conclusions of the theoretical analysis. The work can also offer the information for improving the stability of the system effectively.