Abstract: A mobile ad hoc network is a wireless network that is formed automatically by a collection of mobile nodes.It is particularly vulnerable due to its characteristics of dynamic changing topology.Intrusion detection technology can effectively protect mobile ad hoc networks from attack.In this paper, the existing hierarchical approach are analyzed and a zone-based hierarchical IDS is proposed.The new approach can prevent duplicate monitoring on nodes and small cluster fragments.Moreover, it can decrease the computional complexity and the bandwidth consumption, The stability of cluster heads is improved and resources consumption of cluster heads election is reduced.