Summary In this paper, a new stochastic framework is proposed to study expansion planning problem of energy storage systems in microgrids, which contain renewable resources and responsive loads. Fluctuations on wind and solar generation, microgrid islanding capability and load's variations have considered as sources of uncertainties, which are modelled by sets of scenarios. Reserve constrains are considered to cover stochastic nature of uncertainties. Optimization problem is prepared as a two‐stage stochastic programming. Due to presence of discrete and continues variables, mathematical formulations of optimization model are implemented as a mixed integer linear programming (MILP) problem, which are solved by CPLEX solver in GAMS software. A distribution network consist of 33 buses with some responsive loads is selected as microgird to examine the effectiveness of the proposed approach. Results demonstrate that installing storage devices in microgrid cause cost reduction while improving system adequacy. The proposed model can be utilized as a planning tool for microgrids operators, which want to make decisions on choosing different types of storages and allocate them in optimum buses, considering stochastic contingencies and demand response programs.