<p>A reduction method of engine sound disturbance for on-board power spectral measurement of the tire sound is proposed and its validity is evaluated. The proposed method employs two microphones; one is mounted in the engine compartment, and the other in the wheel well of the vehicle. At first, the sound propagation characteristics between the two microphones are discussed. It is shown that the complex coherence between the engine sounds obtained with two microphones are very low. On the other hand, the power coherence takes about 0.4 within the frequency range from 0 to 15 kHz. These imply that the sound does not propagate linearly between the two microphones, but the power spectra of microphones correlate with each other. Next, the power spectra of the sound measured with each microphone are calculated and the power spectrum of the tire sound is extracted by the calculation with the two spectra based on a pre-identified transfer function between the two microphones. The calculated power spectrum of the tire sound depends on the driving conditions of the engine and its large variance is not acceptable. This may be caused by the error of the pre-identified transfer function. Then, the modified transfer function is determined and the power spectra are calculated again. As a result, the variance of the power spectra reduces from previous results and thus, the validity of the proposed method is confirmed.</p>