Symbiotic Organisms Search (SOS) is a brand new and effective metaheuristic optimization algorithm. This paper proposes the SOS algorithm to solve the Economic Load Dispatch (ELD) problem with valve-point effect, which is one of the essential optimization problems in modern power systems. The proposed algorithm is tested on five different test cases consisting of 3-machine 6-bus, IEEE 5-machine 14-bus, IEEE 6-machine 30-bus, and 13- and 40-unit test systems both with transmission loss and without transmission loss. These test cases show that SOS is able to converge on the global optima, successfully. Moreover, results obtained from the proposed algorithm are compared through different methods used in solving the ELD problem existing in the literature. According to these results, SOS produces the best values among all methods. (C) 2018 Sharif University of Technology. All rights reserved.