Abstract: Radiation efficiency of 4100QB diesel engine gear-cover was investigated by using the Discrete Calculation Method (DCM). Through comparing the radiation efficiency caused by two different exciting methods, exciting the installing steel plate and exciting on gear-cover, the influence of exciting point on radiation efficiency of gear-cover was discussed, and the experimental results of three gear-covers showed that the stiffness affect the radiation efficiency significantly. The stiffness of gear-cover shrinks the high efficiency range within 600 Hz to 1 000 Hz, reduces the radiation efficiency, meanwhile, enlarges the high efficiency range within 1 000 Hz to 2 000 Hz, and increases the radiation efficiency. The paper shows that the changing of structure of thin cover parts should be taken into account when solving the noise problem.