The sizing and techno-economical optimization of a stand-alone micro-grid is presented in this paper . This paper proposes Particle Swarm Optimization (PSO) for solving this minimization problem. In this paper, the problem is formulated as a nonlinear integer minimization problem which minimizes the sum of the total capital, operational and maintenance and replacement cost of DERs, subject to constraints such as energy limits of each DER. The micro-grid considered in this paper consists of wind turbine(WT), photovoltaic array (PV), micro turbine (MT) and battery storage. The aim of present paper is to find the optimum size of component in microgrid, able to fulfill the energy requirement of a given load with lowest cost and to analyze the impact of different parameters on the system’s size. The proposed methodology was used to design micro-grid for north-west of Iran. The simulation studies have shown that the proposed methodology provides excellent convergence and feasible optimum solution for sizing of islanded micro-grids using particle swarm optimization. In this paper, some notions of reliability indices are are calculated in order to evaluate the performance of a microgrid, and the effect of reliability on total cost of micro-grid is evaluated