The　vehicular　ad　hoc　network　(VANET)　has　been　considered　as　one　of　the　most　prominent　technologies　for　improving　the　efficiency　and　safety　of　modern　transportation　systems.　However,　VANET　will　present　a　unique　range　of　challenges　and　opportunities　for　security.　In　particular,　key　management,　as　the　footstone　to　build　a　practical　security　framework　in　VANET,　becomes　a　research　hotspot.　In　this　paper,　we　investigate　key　management　based　on　blockchain　for　VANET.　We　first　propose　an　efficient　decentralized　key　management　mechanism　for　VANET　with　blockchain　(DB-KMM)　to　automatically　realize　the　registration,　update　and　revocation　of　user　&　x2019;s　public　key.　At　the　same　time,　we　present　a　lightweight　mutual　authentication　and　key　agreement　protocol　based　on　the　bivariate　polynomial.　Then,　we　analyze　the　security　of　DB-KMM　in　the　universally　composable　framework　and　show　that　the　mechanism　can　prevent　the　typical　attacks　including　internal　attacks,　DoS　attacks,　public　key　tampering　attacks　and　collusion　attacks.　Finally,　we　analyze　the　performance　of　the　proposed　scheme　through　experiments　and　simulation.　Experiment　results　show　that　DB-KMM　has　better　performance　than　the　existing　schemes　in　terms　of　communication,　storage,　computation　overhead　and　latency.