Distributed generation (DG) is an approach that employs small scale technologies (modular and renewable resources) to produce electricity close to load centers in capacities that ranges from a kilowatt to about 100 MW. Integration of DG units into the electrical distribution network is an efficient and credible way to relieve different issues such as technical, environmental and economical problems. However, improper planning of DG may lead to negative effects into the distribution systems. This study presents an efficient methodology based on water cycle algorithm (WCA) to find the optimal location and size of DG with different types along radial distribution network (RDN) without violating the system constraints. The multi objective function is adopted to simultaneously minimize network power loss and enhance static voltage stability. Moreover, the proposed WCA is used to simultaneously have the optimal topology of RDN and get the optimal DG placement and size in order to minimize power losses and study the improvement in both voltage profile and voltage stability index profile. Loss sensitivity factor (LSF) index is used to get the candidate buses so that reducing the search space for DG allocation. The graph theory is presented to get the fundamental loops that used to reduce the search space for distribution network reconfiguration (DNR) by limiting the infeasible configurations during the optimization process. The proposed method has been applied on two different RDN with different scenarios. The obtained results verify the effectiveness of the proposed algorithm in comparison with previous works in literature. Finally, WCA is combined with firefly optimization algorithm (FA) to develop a new hybrid optimization technique that combines the strength points of each algorithm to optimize for DG allocation and sizing .The performance of the proposed new hybrid algorithm is compared with the basic WCA and FA algorithms in terms of the solution quality.