MANET (Mobile Ad Hoc Network) is a type of ad hoc network that can change locations and configure itself, because of moving to nodes. As MANETs are mobile in nature, they use wireless connections to connect various networks without infrastructure or any centralized administration. Open medium, dynamic topology, distributed cooperation are the characteristics of MANET and hence ad hoc networks are open to different types of security attacks. A Grey hole is a node that selectively drops and forwards data packets after advertises itself as having the shortest path to the destination node in response to a route request message. Our mechanism helps to protect the network by detecting and reacting to malicious activities of any node. The results enable us to minimize the attacks on integrated MANET-Internet communication efficiently. The simulation will be carried out by using network simulator tool so as to address the problem of detection & prevention of gray hole attack in mobile ad-hoc network. In this thesis uses Particle swarm optimization(PSO).Which monitors by changing its values because of ad-hoc nature, converges then it change its value infinite and prevent the node to send packet.