Wireless ad hoc networks are composed of independent nodes, managing the network without any infrastructure. The connection between nodes is provided by sending packages and by trusting to each other. Therefore, using efficient and certain protocols is the essential requirement of these networks. Due to special characteristics of wireless ad hoc networks such as lack of a fixed infrastructure, these protocols are exposed to many attacks. One of these attacks is black hole attack. In this attack, the node running this attack attracts data packages by sending a false response to source node and destroys them. In this paper, the effect of black hole attack on DSR and AODV routing protocol in wireless ad hoc network has been studied. The results simulated with NS2 simulator software show the weaknesses and advantages of these protocols when there are black hole nodes in the network.