The objective of this project is to analysis the impact of black and wormhole attack in manet.Mobile ad hoc network (MANET) is a self-configuring network that is formed automatically via wireless links by a collection of mobile nodes without the help of a fixed infrastructure or centralized management. The mobile nodes allow communication among the nodes outside the wireless transmission range by hop to hop and the forward packets to each other. Due to dynamic infrastructure-less nature and lack of centralized monitoring points, the ad hoc networks are vulnerable to attacks. The network performance and reliability is break by attacks on ad hoc network routing protocols. AODV is a important on-demand reactive routing protocol for mobile ad hoc networks. There is no any security provision against a “Black Hole” and “Wormhole” attacks in existing AODV protocol. Black hole nodes are those malicious nodes that conform to forward packet to destination. But they do not forward packet intentionally to the destination node. The black hole nodes degrade the performance of network eventually by participating in the network actively. The propose watchdog mechanism detect the black hole nodes in a MANET. This method first detects a black hole attack in the network and then provide a new route to this node. In this, the performance of original AODV and modified AODV in the presence of multiple black hole nodes is find out on the basis of throughput and packet delivery ratio. In a wormhole attack, intruders tunnel the data from one end of the network to the other, leading distant network nodes to trust they are neighbors’ and making them communicate through the wormhole link.