Radiation impedance of a vibrating plate on the surface of a visco-elastic medium such as marine sediment varies with the viscoelastic properties of the medium. Therefore, it is possible to predict the viscoelastic properties using the characteristics of the radiation impedance. In this paper, the characteristics of radiation impedance of a circular vibrating plate on the surface of viscoelastic media were obtained by numerical analysis. The results showed that the values of ka (k is the wave number, a is the radius of a circular vibrating plate) at which the imaginary part of the radiation impedance becomes zero can be related to the ratio of the shear wave velocity to the longitudinal wave velocity, that is, the Poisson's ratio. Next, measurements of radiation impedance for three kinds of urethane rubbers and two kinds of dry beach sands were done, and the measured values were compared to the calculated values.