In order to use viscoelastic materials efficiently for noise and vibration control, or th qualify newly developed materials, knowledge of the Young s modulus and loss factor is essemtial. These material properties, the so-called complex Young s modulus, are frequently treated as dynamic charicteristics because of their dependence upon the frequency. Many techniques have been developed and verified for measuring complex Young s modulus of viscoelastic materials. Among them, the impedance method is preferable in order to obtain the frequency information in detail. In this method, a cylindrical or prismatic specimen is excited into longitudinal harmonic vibration at one end, the other being fixed, and the resulting force is measured at the driving or fixed end. The amplitude ratio of the two signals and phase angle between them are then used to compute the material properties using various mathematical models. In this paper, the impedance method is investigated theoretically and experimentally. A way to determine the specimen geometry which is most appropriate for the identification of complex Young s modulus using the lumped mass model is presented and discussed. Then experimental results supporting the theoretical predictions are presented.