The distributed generation (DG) locations have significant impacts on network reliability and loss. This paper presents a method for optimal DG allocation and sizing in distribution systems in order to reduce network loss and improve system reliability at the lowest cost. For this purpose, a multi-objective cost function considering active power loss, reliability index and DG cost is used to find the optimal size and location of DG units. Also, a load duration curve including four load levels with different weighting factor is considered. Moreover, the optimization technique that is based on Genetic Algorithm (GA) is implemented. In this paper, ECOST index is considered for reliability evaluation in the cost function and also, the effects system reconfiguration, is also considered for reliability assessment. To evaluate the proposed algorithm, the IEEE 33-bus and 69-bus test systems are employed and the results obtained reveal the effectiveness of proposed method.