There are several routing protocols that have been proposed for the possible deployment of MANETs in many fields like military, government and commercial applications. While the routing aspects of MANETs are already well understood but the research activities about the security in MANETs are still at their beginning. This paper focuses on the performance investigation of reactive and proactive MANET routing protocols, namely, AODV and OLSR, under Black-Hole Attack. The performance evaluations of metrics chosen are end to end delay, retransmission attempts, network load and throughput, when a percentage of nodes misbehave. It is evaluated that it is difficult to detect Black Hole attack, on the basis of the performance of the network.