In the case in which a rectangular plate is excited sinusoidally, the estimation of vibrational energy flow by sound energy is investigated. In this paper, the analysis of both vibration of the rectangular plate and the radiated sound field is performed at each mode of vibration. Furthermore, the changes of vibrational energy flow and sound energy flow are analyzed under the condition of a different damping coefficient. In evaluating the vibrational energy and the sound energy, the structural and sound intensity are used under consideration of three-dimensional vibrational energy and sound energy flow. By discussing the distribution of the vibrational and sound energy and the ratio of sound energy to the vibrational energy, etc., the possibility of estimating the vibrational energy flow by the sound energy is also indicated.