Three　efficient　rate　allocation　schemes　for　the　MPEG-4　fine　granular　scalability　(FGS)　enhancement　layer　with　constant　quality　are　proposed　in　this　paper.　With　the　distortion　measured　using　the　mean　squared　error　(MSE)　or　peak　signal　noise　ratio　(PSNR)　metric,　the　rate-distortion　(R-D)　curve　of　the　MPEG-4　FGS　enhancement　layer　exhibits　piecewise　characteristics.　For　each　piecewise　region,　a　first-order　or　a　second-order　model　is　adopted　to　estimate　the　actual　R-D　curve.　Based　on　these　piecewise　models,　three　optimal　rate　allocation　schemes　are　proposed　to　allocate　an　appropriate　number　of　bits　to　the　enhancement　layer　of　each　frame.　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.