In　H.264/AVC,　a　novel　criterion　named　the　rate　distortion　optimization　(RDO)　is　employed　to　select　the　optimal　coding　modes　for　each　macroblock　(MB)　within　the　intra　prediction　coding　pictures,　which　can　achieve　a　high　compression　ratio　while　leading　to　a　great　increase　in　the　complexity　and　computational　load　unfortunately.　In　this　paper,　a　fast　mode　decision　algorithm　based　on　integer　transform　and　adaptive　threshold　is　proposed.　Before　the　intra　prediction,　integer　transform　operations　on　the　original　image　are　executed　to　find　the　directions　of　local　textures.　According　to　this　direction,　only　a　small　part　of　the　possible　intra　prediction　modes　are　tested　at　the　first　step.　If　the　suim-nation　of　differences　(SAD)　of　the　reconstructed　block　corresponding　to　the　best　mode　is　smaller　than　an　adaptive　threshold,　calculation　is　terminated.　Otherwise,　more　possible　modes　are　needed.　Simulations　show　that　the　fast　mode　decision　algorithm　proposed　in　this　paper　can　accelerate　the　speed　of　intra　picture　coding　significantly　only　with　a　negligible　PSNR　loss　or　a　bit　rate　increment.