Lithium　Niobate　(LN)　and　Quartz.　crystal　are　good　piezoelectric　crystals,　they　can　be　used　bases　Of　Surface　acoustic　wave　(SAW)　devices.　In　this　paper,　SAW　elemental　equation　and　corresponding　mechanics　boundary　condition　equation　are　deduced.　For　the　above　two　crystals,　SAW　velocities　are　calculated　systematically,　and　curves　of　the　acoustic　reciprocal　velocity　are　drawn　using　a　new　circle　iterative　method　in　decoupling　acoustic　sagittal　plane,　that　is　yz　plane.　Calculation　results　lay　a　good　theoretical　base　for　design　of　SAW　devices　and　have　research　signification　and　practice　value.