In the future, skyscrapers or large structures will become lighter and more flexible. In order to clarify the aerodynamic behavior of such structures by wind tunnel test, flexible, low-density square prism models were developed using polyurethane foam. In this report, mechanical properties and bending vibration characteristics of the models are investigated by material tests and vibration tests. The results show that the models can be treated as uniform and elastic within the range of the deformation occurred by the across-wind aerodynamic vibration. It is shown that, bending vibration characteristics can be evaluated using the Bernoulli-Euler beam theory.