This study addresses distributed generation allocation for power loss and voltage deviation reductions in distribution systems. To reach the mentioned benefits determining the optimal capacity and location of the DGs is important. Since problem formulation includes several individual objectives, hence an optimization algorithm i.e., Improved Particle Swarm Optimization (IPSO) algorithm is used to allocate muli-DG units in radial distribution systems. To verify the effectiveness of the proposed algorithm in finding best solutions, IEEE 33 bus standard system and a 22 bus practical system of Tehran City are selected. Simulation results indicate the efficacy of the proposed algorithm in DG sizing and sitting.