Mobile Adhoc Networks (MANETs) are dynamic in nature. Any nodes can join and leave the network at any time. Hence any type of intruders can attack the communication at any time, especially the routing mechanism between the nodes. In this study, we study and understand two types of attacks which cause more damage to the routing performance of MANET; the attacks are Black Hole attacks and Gray Hole attacks and compare the impact of these attacks on MANET. "Send Fake Route Reply to the nodes" type of attack is used to understand the behavior of these two types of attacks. Existing AODV protocol is modified in order to study these types of attacks in MANET. Performance evaluation of the proposed method is carried out using NS-2. In the presence of these attacks the network performance degrades for various network attributes. The performance of MANET under attack is thoroughly investigated, by applying it on various network parameters with various node densities. Not only had that which attack causes more damage to the environment also studied.