To analyze the characteristics of engine exhaust mufflers by the acoustic theory is necessary for understanding the action of mufflers and for designing the mufflers. In many cases, the " Transmission Loss" is used in the analysis, but TL is not suitable for above purposes. In this paper the characteristics of the mufflers is analyzed by means of the four terminal constants of whole muffler system. After some approximation, the characteristics of engine exhaust mufflers is shown by the "D" in the four terminal constants. 20log│D│ can be easily constructed with the characteristics of each element of the muffler system. The analysis is done under the conditions of no loss and no gas-flow, so the result is approximate. But it is very useful for understanding the characteristics of the muffler system or each element of the system and for designing the muffler system.