MANET stands for Mobile Ad hoc Network. It is a robust infrastructure less wireless network. malicious vehicles can degrade the network performance by triggering some security attack. VANET are self-configuring networks composed of a collection of vehicles and elements of roadside structure linked with each other without requiring any infrastructure, sending and receiving information of current traffic situation. These are used for the communication among the mobile vehicles. It has some security issues like attacks, authentication etc. In this work, a novel technique has been proposed to detect malicious vehicles and isolate Sybil attack from the network. This will help to improve network performance.