It is required that the indoor-noise level of KHST (Korean High Speed Train) should be lower than 66 dB(A) at 300 km/h. In this study, the indoor-noise level of KHST has been predicted to determine the maximum allowable sound power levels of major noise sources. It is found that the indoor-noise requirements for KHST can be met by increasing the transmission losses of the floor and side-wall structures as well as by lowering the sound power levels of the major noise sources.