Based on the theory of the mechanics of incremental deformations, guided wave propagation in multilayered plates under initial stresses in the thickness and wave-propagation directions is investigated. The Legendre polynomial series method is used to solve the coupled wave equations and its convergence is discussed along with numerical examples. The effects of initial stresses on Lamb-like waves and SH waves are analyzed, and numerical results show that they are quite distinct. The frequency variation is not linear with the increase of the initial stresses in both directions. The effects of the initial stress in the thickness direction are very different from those in the longitudinal direction.