Lithium　Niobate　(LN)　crystal　has　excellent　piezoelectric　properties,　it　can　be　used　as　substrates　of　surface　acoustic　wave　(　SAW)　devices,　for　example　delay　line,　filter,　oscillator,　convolver　and　so　on.　Intrinsic　SAW　velocity　is　determined　by　the　internal　property　of　crystals.　In　this　paper,　SAW　acoustic　basic　equation　group　and　SAW　mechanical　boundary　condition　equation　group　are　deduced.　Intrinsic　SAW　velocities　are　calculated　systematically　using　a　circle　iterative　method　in　zx,　xy　and　yz　three　coordinate　planes　of　LN　along　different　directions.　Stiffness　coefficient　of　piezoelectric　crystal　can　be　changed　by　piezoelectric　effect　and　it　is　named　as　piezoelectric　modified　stiffness　coefficient.　Reciprocal　velocity　Curves　of　LN　in　the　three　coordinate　planes　using　the　non-modified　stiffness　coefficients　and　the　modified　stiffness　coefficients　are　drawn.　Many　characters　of　the　SAW　device　are　related　to　the　acoustic　velocities,　so　research　results　lay　a　solid　base　for　design　and　manufacture　of　the　SAW　device.　It　has　theoretical　significance　and　practical　value.