Rate　allocation　algorithm　for　the　MPEG-4　Fine　granular　scalability　(FGS)　enhancement　layer　is　an　important　tool　in　MPEG-4　FGS-based　video　streaming　applications.　Based　on　the　fact　that　the　Rate-distortion　(R-D)　curve　of　the　MPEG-4　FGS　enhancement　layer　exhibits　piecewise　characteristics,　and　each　piecewise　region　of　the　curves　corresponds　to　a　different　bit　plane　in　the　enhancement　layer,　a　first-order　or　a　second-order　model　is　adopted　to　estimate　the　actual　R-D,　curve　for　each　piecewise　region.　Then,　three　efficient　rate　allocation　methods　are　proposed　to　allocate　an　appropriate　number　of　bits　to　the　enhancement　layer　of　each　frame　so　as　to　achieve　the　constant　quality.　Experimental　results　show　that,　compared　to　the　average　rate　allocation　method　and　the　exponential　model　method,　our　proposed　algorithms　can　achieve　a　much　constant　quality　in　the　reconstructed　video.　Complexity　analysis　also　shows　that　our　proposed　methods　are　efficient　and　require　little　computation　at　the　video　server.　Therefore,　they　are　practical　for　video　streaming　applications.