We　consider　the　problem　of　cooperative　spectrum　sharing　among　primary　users　(PUs)　and　secondary　users　(SUs)　in　cognitive　radio　networks.　In　our　system,　each　PU　selects　a　proper　set　of　SUs　to　serve　as　the　cooperative　relays　for　its　transmission　and　in　return,　leases　portion　of　channel　access　time　to　the　selected　SUs　for　their　own　transmission.　PU　decides　how　to　select　SUs　and　how　much　time　it　would　lease　to　SUs,　and　the　cooperative　SUs　decide　their　respective　power　levels　in　helping　PU＇s　transmission,　which　are　proportional　to　their　access　times.　We　assume　that　both　PUs　and　SUs　are　rational　and　selfish.　In　single-PU　scenario,　we　formulate　the　problem　as　a　noncooperative　game　and　prove　that　it　converges　to　a　unique　Stackelberg　equilibrium.　We　also　propose　an　iterative　algorithm　to　achieve　the　unique　equilibrium　point.　We　then　extend　the　proposed　cooperative　mechanism　to　a　multiple-PU　scenario　and　develop　a　heuristic　algorithm　to　assign　proper　SUs　to　each　PU　considering　both　performance　and　fairness.　The　simulation　results　show　that　when　the　competition　among　SUs　is　fierce,　the　performance　gap　between　our　heuristic　algorithm　and　the　optimal　one　is　smaller　than　3%.　Copyright　©　2013　John　Wiley　&　Sons,　Ltd.