This paper presents the three-dimensional analysis of spurious vibrations in rectangular ATcut quartz plates, based on exact equations of motion. According to the classical concept, threedimensional displacements are described with linear combination of the eigenmodes guided along the X crystal a.xis. Guided eigenmodes are determined by applying the one-dimensional finite element method to the two-dimensional problem, and their amplitudes are det,ermined using the variational technique for the three-dimensional problem. By means of t,he present method, we can perform the tllree-dimensional analysis of spurious vibrations with a high degree of accuracy within a reasonable computation time. As numerica.1 examples, frequency spectra for rectangular AT-cut quartz plates are computed and compared with Koga's experimental data. A convergence study is carried out for the mode for which the frequency spectra are not, satisfactory, and corresponding vibrational patterns are also shown. 518 0-7803-3309-8/96 $5.00 0 1996 IEEE