Abstract: An equivalent method of resonant frequence is provided to determine the magnitude of the required extra mass when an acceleration transducer is applied to measuring the vibrationsof an engine instead of velocity transducer without changing the original technical specifications.The effect of the transducer mass on the dynamic characteristics of the rear bearing-case system is investigated.The experiments show that the first several orders of the resonant frequences and displacement mobility peak values vary to a great extent because of great difference between two transducer masses when the velocity transducer is replaced by the acceleration transducer in measuring the vibrations of the system.