Honeycomb panel is consisted of 3 layers that are double-faced sheets and honeycomb-shaped core. It is highly desirable for ship, railway, and aerospace industry. The reason is that honeycomb panel excels in strength and in its weight. However in terms of insulation, it is a little bit insufficient to commonly use sandwich-panel. In this paper, Moor’s theory is used to predict sound transmission loss (STL). The theory is assumed that core layer is homogeneous orthotropic. And to calculate STL, it is evaluated in terms of the symmetric and anti-symmetric panel impedances, and the characteristic impedance of air. After that predicted data are compared with experiment data.