The most important danger for power system security are cascading failures and blackouts during contingency situations. In order to reduce the risk of cascaded outage and blackout, load shedding has been considered as a preventive scheme. This paper presents an application of the heuristic optimization algorithm, particle swarm optimization (PSO) algorithm to find an optimal load shedding strategy for radial distribution systems during an overload contingency. The radial distribution systems are the final link of the interconnection between power systems and the consumers with unidirectional power flows. Overload contingency in the radial distribution systems with and without installed distributed generations (DGs) are the two cases considered in this paper. By the introduction of the distributed generations, the electrical distribution system has a locally looped system and bidirectional power flows. The main objective of the proposed algorithm is to minimize the sum of curtailed load based on their assigned degree of importance and system losses within the operational and security constraints of the system. The proposed method has been tested on 12-bus, 33-bus and 69-bus radial distribution systems. The feasibility of the proposed algorithm has been established and compared with genetic algorithm (GA) in terms of convergence characteristics and solution quality over realistic test systems considered. © 2014 Praise Worthy Prize S.r.l. - All rights reserved.