Based on Mindlin plate theory, the frequency spectrum of face shear mode in AT-cut quartz plates and its frequency temperature behavior are studied. The results of numerical simulation are compared with experiment data. Both exact three-dimensional plate equations and Mindlin first-order plate equations are used to calculate the spectrum respectively, which can help engineers to decide suitable dimensions of AT-cut quartz plate to avoid the couple of thickness-shear and face-shear. The temperature behavior characteristic is quite an important characteristic of quartz resonator. In the present paper, the frequency temperature behavior of face-shear mode is also studied. Linking the frequency spectrum and temperature behavior results can enable engineers not only to get suitable dimensions of AT-cut quartz plate at room temperature but also to get suitable dimensions at certain temperature range. The measurement data for rectangular, AT-cut quartz plates is also demonstrated.