With the increasing number of ongoing replacement of conventional power plants by distributed energy resources (DERs), the power system becomes more vulnerable to operational disturbance. It is necessary to accurately estimate the inertia of the DERs-dominant systems, based on which the system can be tuned and stabilized after disturbances. A disturbance-induced inertia estimation method is introduced in this paper for identifying the inertia of microgrid systems with power-electronics-interfaced DERs. This method calculates the total power change in response to a disturbance in the system by selecting appropriate time intervals for the Rate Of Change Of Frequency (ROCOF) curve. The numerical results on a typical microgrid system have validated the feasibility of the presented method in estimating microgrid's inertia.