Mobile　Ad-hoc　Networks　(MANETs)　are　considered　as　the　most　promising　terminal　networks　in　future　wireless　communications　and　characterized　by　flexibility,　fast　and　easy　deployment,　which　make　them　an　interesting　technology　for　various　applications.　Group　communication　is　one　of　the　main　concerns　in　MANETs.　To　provide　the　secure　group　communication　in　wireless　networks,　a　group　key　is　required　so　that　efficient　symmetric　encryption　can　be　performed.　In　this　paper,　we　propose　a　constant-round　group　key　agreement　scheme　to　enable　secure　group　communications,　which　adopts　the　Identity　Based　Broadcast　Encryption　(MBE)　methodology.　When　a　new　Ad-hoc　network　is　constructed,　the　suggested　scheme　requires　no　message　exchange　to　establish　a　group　key　if　the　receivers＇　identities　are　known　to　the　broadcaster,　which　is　an　advantage　that　outperforms　most　of　the　existing　key　agreement　schemes.　The　proposed　scheme　can　build　a　new　group　and　establish　a　new　group　key　with　ease　when　member　joins　or　leaves.　In　addition,　our　scheme　is　efficient　in　computation　and　only　one　bilinear　pair　computation　is　required　for　group　members　to　obtain　his/her　session　key.　A　highlight　property　of　the　scheme　is　that　communication　cost　remains　unchanged　as　group　size　grows.　Furthermore,　we　show　that　the　new　scheme　is　proved　secure　without　random　oracle.　Thus,　the　scheme　can　not　only　meet　security　demands　of　larger　mobile　Ad-hoc　networks　but　also　improve　executing　performance.