Abstract: 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.