In this work, an optimal design framework for the isolated microgrid system is presented, which contains two-level optimization for the configuration and operation of both the distributed renewable resources (DRR) and controllable loads. The controllability of special loads is discussed and modeled by the investment cost function and simplified utilization strategy. Moreover, a particle swarm optimization (PSO) algorithm is used for the solution of the proposed optimal design problem. Test results for an isolated microgrid using PV, wind turbine and batteries shows that as the controllability of loads is considered, the penetration of DERs as well as overall benefit of the microgrid can be enhanced remarkably.