Compared　with　other　existing　video　coding　standards,　H.264/AVC　can　achieve　a　significant　improvement　in　compression　performances.　A　robust　criterion　named　the　rate　distortion　optimization　(RDO)　is　employed　to　select　the　optimal　coding　modes　and　motion　vectors　for　each　macroblock　(MB),　which　achieves　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　for　H.264/AVC　intra　prediction　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　for　RDO　calculation　at　the　first　step.　If　the　minimum　mean　absolute　error　(MMAE)　of　the　reconstructed　block　corresponding　to　the　best　mode　is　smaller　than　an　adaptive　threshold　which　depends　on　the　quantization　parameter　(QP),　the　RDO　calculation　is　terminated.　Otherwise,　more　possible　modes　need　to　be　tested.　The　adaptive　threshold　aims　to　balance　the　compression　performance　and　the　computational　load.　Simulation　results　with　various　video　sequences　show　that　the　fast　mode　decision　algorithm　proposed　in　this　paper　can　accelerate　the　encoding　speed　significantly　only　with　negligible　PSNR　loss　or　bit　rate　increment.　©　2007　Springer-Verlag　London　Limited.