<p>Although some third-order shear deformation plate theories, including Levinson plate theory (LPT) and Reddy plate theory (RPT), are available, their computational complexity limits their application. Based on LPT, a single-variable third-order shear deformation theory, named the simplified LPT, is proposed by requiring vanishing torsional deformation along the thickness direction. The LPT is described by a sixth-order partial differential equation (PDE), whereas the simplified LPT is described by a fourth-order PDE, similar to that of classical plate theory (CPT). Following the CPT, the twisting moments are converted into shear forces in the boundary conditions of the simplified LPT. To further demonstrate the effectiveness of the simplified LPT, the free vibration analysis of Lévy-type plates is conducted, where two opposite edges are simply supported (or guided) while the remaining two edges are subject to arbitrary boundary conditions. Using the Lévy approach, the natural frequencies based on the CPT, Mindlin plate theory, LPT, RPT, and simplified LPT are compared. The numerical results indicate that the simplified LPT is effective and can be applied to thin, moderately thick, and thick plates. Consideration of shear deformation brings the results of the simplified LPT closer to those of LPT and RPT.</p>