Mobile Ad hoc Network is a collection of wireless mobile nodes that dynamically creates network without a fixed infrastructure. However, all the characters make the security problems, such as Blackhole, Grayhole attacks. In this article, we propose Valid Route Authentication mechanism (VRA) for security, and integrating VRA into the discovery route process of AODV protocol. Improved protocol named VRA-AODV which can detect Black and Grayhole attacks. Using NS2, we compare the performance of AODV and improved protocols in the mobility nodes topology under attacks. Simulation results show that improved protocol has better performance AODV based on packet delivery ratio, network throughput and routing load metrics.