A　mobile　Ad　Hoc　network　is　a　multi-path　autonomous　system　comprised　of　many　mobile　nodes　with　wireless　transmission　capability.　Although　there　is　a　trend　to　adopt　ad　hoc　networks　for　commercial　uses　due　to　their　unique　properties.　One　main　challenge　in　design　of　these　networks　is　their　vulnerability　to　security　attacks.　In　this　paper,　we　first　review　current　research　in　key　management　in　mobile　Ad　Hoc　networks.　Then　by　considering　the　characteristics　of　such　networks　in　which　free　movement　of　mobile　nodes　can　lead　to　frequent　topological　changes,　especially　network　separation　and　convergence,　we　propose　an　key　management　mechanism　based　on　a　combination　of　techniques,　such　as　hierarchical　topology　structure,　virtual　CA　(certification　authority),　offline　CA,　certificate　chain,　and　proactive　measurement,　and　show　that　the　proposed　scheme　could　improve　the　security　of　mobile　ad　hoc　network　in　the　areas　of　expandability,　validity,　fault　tolerance　and　usability.