The most important perils for power system security are cascading failures and blackouts during contingency situations. In order to reduce the risk of cascaded outages and blackout, load shedding has been considered as a preventive scheme. This paper presents a novel application of the heuristic optimization algorithm, improved gravitational search algorithm (IGSA) to find an optimal load shedding strategy for radial distribution systems during an overload contingency. Overload contingency in the radial distribution systems without and with installed distributed generations (DGs) are the two cases considered in this paper. The proposed method has been tested on IEEE 33-bus radial distribution system. The feasibility of the proposed algorithm has been validated with results reported using Genetic Algorithm (GA) in terms of quality of the solution over realistic test systems considered. © 2017 IEEE.