In　order　to　select　optimum　cut　direction　of　the　substrates　in　designing　devices,　a　concept　of　piezoelectric-modified　stiffness　coefficients　is　put　forward　and　it　is　calculated　for　the　quartz　and　LN　crystal.　Intrinsic　SAW　basic　equations　and　mechanics　boundary　conditions　equations　are　deduced　from　character　equation　of　crystal.　Using　a　new　circular　iterative　method,　non　piezoelectric-modified　and　piezoelectric-modified　intrinsic　SAW　reciprocal　velocity　curves　are　calculated　in　three　coordinate　planes　for　the　quartz　and　LN　crystals.　Research　results　indicate　that　intrinsic　SAW　velocities　are　quickened　due　to　the　piezoelectric　effect.　There　is　internal　relationship　between　the　SAW　properties　and　point　group　symmetries　of　the　crystal.