Virtual Synchronous Generator(VSG) is proposed as a control strategy applied to the inverter for power system stability enhancement by imitating the behavior of a synchronous generator. In order to study the stability of a VSG, a small-signal state space model of the VSG, including the inverter, LCL filter and the control system, is built. The correctness of the small-signal model is verified by a Simulink model. The modes of the system are identified and the sensitivities of some eigenvalues are calculated. To study the dynamic interaction between the VSG and power distribution networks, the impedance-based stability of the VSG is analyzed.