The operation of a distribution system in the presence of distributed generation systems has someadvantages and challenges. Optimal sizing and siting of DG systems has economic, technical, andenvironmental benefits in distribution systems. Improper selection of DG systems can reduce theseadvantages or even result in deterioration in the normal operation of the distribution system. DGallocation and capacity determination is a nonlinear optimization problem. The objective function ofthis problem is the minimization of the total loss of the distribution system. In this paper, the ImprovedHarmony Search (IHS) algorithm has been applied to the optimization problem. This algorithm has asuitable performance for this type of optimization problem. Active and reactive power demands of thedistribution system loads are dependent on bus voltage. This paper verifies the effect of voltagedependent loads on system power characteristics. The load model has an inevitable impact on DGsizing and placement. The proposed algorithm implemented and tested on 69-bus distribution systemsand the impact of voltage dependent load models are demonstrated. The obtained results show that theproposed algorithm has an acceptable performance.