Theoretical analyses and experimental studies have been carried out on the non-linear hydroelastic vibration of a cylindrical tank with an elastic bottom. In this paper, non-linear axisymmetric free vibration analysis of the bottom plate of the tank, coupled with liquid contained, is presented by means of the Ritz averaging method. In the analysis, the effect of an in-plane force in the plate due to the static liquid pressure is taken into account. The effect of liquid contained on the non-linearity of the backbone curve of the principal resonance of both sloshing- and bulging-type responses was clarified, and it was found that with increase of the liquid height, the non-linearity with a hard-spring type of bottom plate decreased in degree, and in fact became close to linear characteristics. The effect of elastic stiffness of the foundation on the bottom plate response was also clarified.