The Moss Rosenberg Verft spherical tank type L. N. G carrier was built in M. H. I s Nagasaki shipyard recently. She has tall superstructure in the after body. It was considerd necessary to control vibration level, and vibrations study has been performed by modal analysis and Finite Element Method. The exciter test and measurement at sea trial has been carried out for confirmation on completion of the ship, and following items were confirmed (a) Present calculation method is useful for estimation of dynamic characteristic of ship. (b) Vibration level of L. N. G. C was controlled satisfactorily as we approched and expected.