The microgrid is operated in the grid connected mode and the islanded mode according to the upstream grid. Commonly, microgrid operates the grid connected mode. But, when a fault occurs in the upstream grid, it should disconnect and shift into islanded operation mode. It is important to maintain the voltage and frequency constant values in islanded mode. To achieve these management goals appropriately, In this paper, the coordinated control strategy between Diesel generator and BESS(Battery Energy Storage System) is needed. And a fuzzy PI controller is proposed to improve the frequency and voltage control performance of the BESS. The performance of suggested Fuzzy PI Controller and coordinated control were simulated by PSCAD EMTDC.