In this paper, an effective methodology is proposed for the optimal allocation of conventional (Gas turbines) and renewable based distributed generators (solar, wind) in the distribution system (DS) are presented. The objectives are to minimize real, reactive power losses and emission produced by the sources. Initially, the best locations for placement of DGs are identified by voltage stability factor (VSF) concept. The number and size of solar, wind based DGs and gas turbines corresponding to these locations are determined by applying search-based dragonfly algorithm (DFA). The generation uncertainties associated with wind and solar based DGs is effectively modeled by Weibull and beta probability distribution functions (PDF) to determine the exact output power. Two different scenarios, i.e. optimal allocation and the combination of different types DERs in the distribution system is considered in this analysis. The developed method is tested on IEEE 33 and 69 bus distribution systems. The results show its effectiveness in terms of solving respective objective function. © 2019, Gazi University Eti Mahallesi. All rights reserved.