In islanded micro-grid design, a proper Distributed Energy Resource (DER), sizing and effective coordination between resources are important and challenging optimization tasks. The types and sizes of renewable energy sources such as wind turbines, photovoltaic panels, fuel cell and the capacities of battery bank and the other distributed generators must be optimized in islanded micro-grid design. 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. In this paper we use Particle Swarm Optimization (PSO) for solving this minimization problem. The proposed methodology was used to design micro-grid for northwest of Iran. In this paper some notions of reliability are considered for micro-grid, and the effect of reliability on total cost of micro-grid is evaluated. In this paper we assume that micro-grid consists of interruptible and uninterruptible loads.