The optimization of timing gear cover was carried by using Altair Hyperworks, to gain the optimal mass distribution. Hereby, the structural modification was performed. The mode of timing gear cover avoided the frequency range of timing gear train excitation, to reduce the radiated noise. In order to calculate radiated sound power of timing gear cover, the finite element method (FEM) and boundary element method (BEM) were used. The calculating result shows that the radiated sound power on new timing gear cover decrease by 2.4dB, which was validated by the verification test. 