In order to extend the frequency of Surface Acoustic Wave(SAW) filters to a higher band, the principles of SAW filters are analyzed. At first, piezoelectricity and ob-piezoelectricity of piezoelectric film are presented. Then the performances of crystal structure of piezoelectric material are compared among different examples. The electro-mechanical coupling coefficients of important physical quantities are introduced as well as the physical mechanism of SAW transmission. Then the transmission properties of SAW in the piezoelectric material and piezoelectric film are studied in detail. And the transmission properties of SAW in piezoelectric medium and in ob-piezoelectric medium are contrasted. The results indicate that SAW is non-dispersive wave in the piezoelectric material with three directions of particle displacement vectors; while in the piezoelectric film, SAW is dispersive wave, and the piezoelectric film would influence the velocity of SAW.