Abstract: The sound transmission of an incident plane wave in the low-frequency range from air to water through stiffened plates mounted in an infinite baffle is investigated by using the finite element method and the boundary element method. The numerical results show that the influence of the stiffness of a plate on sound transmission is important if the plate's stiffness is large, and is insignificant if the plate's stiffness is small. In the latter case it is the big difference of characteristic acoustic impedance between air and water which dominates the sound transmission. The position of minimum values on the sound transmission loss curve varies with the changes made on plate thickness and the moment of inertia of stiffeners. An increase in the plate thickness and the moment of inertia of stiffeners increases the sound transmission loss, especially while the excitation frequency is below the fundamental natural frequency of the plate.