Quartz　and　lithium　niobate　crystals　have　good　piezoelectric　properties;　they　can　be　used　as　the　substrates　of　delay　line,　filter,　oscillator,　convolver　and　acousto-optic　devices.　In　this　paper,　the　crystal　acoustics　basic　equations　and　mechanics　boundary　conditions　equations　of　the　surface　acoustic　wave　are　deduced.　In　optimum　decoupling　acoustic　sagittal　plane,　that　is　yz　plane,　using　a　new　circle　iterative　method,　surface　acoustic　wave　velocities　for　quartz　and　lithium　niobate　crystals　are　calculated　in　different　directions　systematically,　and　the　curves　of　the　acoustic　reciprocal　velocity　are　drawn.　The　research　results　lay　a　solid　theoretical　base　for　the　design　and　fabrication　of　surface　acoustic　wave　device.