Abstract: A modal analysis model of complex structural acoustic radiation in light fluid based on the use of the finite element method and the boundary element method is proposed for computing modal radiation efficiency, farfield pressure, structural radiated power and radiation efficiency. Numerical examples of the modal analysis of acoustic radiation of plate and stiffened plate are given to test and to compare with the results using Rayleigh integral in the farfoeld. The effects of disturbance location on acoustic radiation of plate are discussed. The characteristics of modal radiation efficiency of plate and stiffened plate are also presented and it is shown that the trait of the modal radiation efficiency of plate with stiffeners is different from plate's.