Impedance is an important parameter to analysis the stability of microgrid. Stability criteria have been developed in terms of source and load impedances for both dc and ac systems, and it is often helpful to have techniques for impedance measurement. First, the theoretical impedance of microgrid is calculated in this paper. Second, a new method of impedance measurement for microgrid is introduced. By injecting an unbalanced line-to-line square wave current between two lines of the ac system, all impedance information in the traditional synchronous reference frame d-q model can be determined. The proposed measurement device also can be used for both ac and dc interfaces. The impedance identification method proposed in this paper greatly reduces online testing time by injecting an unbalanced line-to-line square wave current. Calculation, simulation and laboratory measurements verify the measurement results at last.