Recently, sound or noise radiated from vibrating structures has become a problem of importance in society. Most of the sound radiated from vibrating structures is known to be structure-borne sound. The purpose of this study is to investigate the sound generating mechanism from the viewpoint of both sound energy and vibrational energy. To evaluate the transformation from structural vibration to radiated sound, the sound radiation efficiency is generally used. However, the sound radiation efficiency does not directly express the relationship between structural vibration energy and sound energy. In this study, we propose a definition of energy transformation efficiency which expresses the relationship between structural vibration energy and radiated sound energy. The energy transformation efficiency of a rectangular plate from vibration to sound is calculated from the eigenfrequency, eigenmode and size of the vibrating rectangular plate. As a result, these parameters of the vibrating rectangular plate, which influence radiated sound energy, are clarified by using the proposed energy transformation efficiency.