This paper describes the predicted results of acoustic transmission loss (T.L.) for a motorcycle muffler. First, the T.L. of a prototype muffler with one expansion chamber was obtained by measuring sound levels at the inlet and outlet ports of the muffler by speaker test. T.L. was then calculated by using a three-dimensional Finite-Element Method (FEM) for acoustic fields in the muffler. There was good coincidence between the calculated T.L. and experimentally observed data.Second, T.L. of the prototype muffler while attached to a motorcycle engine was measured. On this step, however, a similarly calculated T.L. using FEM to consider the effect of exhaust gas temperature in the muffler showed differences from the measured one. It was estimated that muffler body vibration sounds may affect the result. A dynamic analysis of the structure was carried out using FEM to obtain the eigen modes of the muffler body. The calculated eigen values and modes were in good agreement with the measured results on the hammering test. The vibration response of the muffler was calculated for each frequency by using modal analysis. As we calculated the acoustic field in the muffler with the mass velocity as the boundary condition, we obtained T.L. of relatively good coincidence between the calculations and the test observations. It became clear as a conclusion that it is necessary to include the effect of structural vibration when predicting the T.L. on the muffler.