Distributed generation (DG) sources are predicated to play major role in distribution systems due to the demand growth for electrical energy. Location and sizing of DG sources found to be important on the system losses and voltage stability in a distribution network. In this paper an efficient technique is presented for optimal placement and sizing of DGs in a large scale radial distribution system. The main objective is to minimize network power losses and to improve the voltage stability. A detailed performance analysis is carried out on 33-bus, 69-bus and 118-bus large scale radial distribution systems to demonstrate the effectiveness of the proposed technique. Performing multiple power flow analysis on 118-bus system, the effect of DG sources on the most sensitive buses to voltage collapse is also carried out.