A rare kind of structural vibration induced by flow was experienced on sea chests of high speed ships. Model experiments in cavitation tunnel revealed the cause of vibration and the mechanism of excitation. The results showed that the free shear layers flowed into the cavity of sea chest were rolled up periodically into vortices and the vortex excitation resonated with the natural frequency of the vibration system composed of the sea chest structure and the water involved in it. Countermeasures reducing the excitation were also obtained by the model test. Energy method was applied for the estimation of the natural frequency of sea chest vibration system. The calculated results by this method were verified to correlate well with the results of vibration test on sea chest model, together with the measured results on actual ship.