In addition to ordinary vibration study in conventional ships, study on the following vibrations is necessary in Moss type LNG carrier in order to avoid damages in structures and equipments caused by critical vibration as a result of resonance of structures with blade frequency. 1) Vibration of tank system including spherical tank, skirt and foundation deck. 2) Vibration of spherical tank, i. e., spherical shell. 3) Vibration of tank cover, dome at upper part of spherical tank, pipe tower in spherical tank, etc. The natural frequency of the vibration in item 3) can be easily estimated, and it can be changed by small modification of design, and that of the spherical tank vibration in item 2) was already obtained by our detailed study so far. As for the vibration of the tank system in item 1), on the other hand, it seems that there is not any established estimating method of the natural frequency and the vibration response. The present papers deal with the study on the vibration of the tank system for a 128, 600 m³ Moss type LNG carrier which we will builds. First, we have made a theoretical calculation on an ideal mathematical model considering coupled vibration of tank system and hull. In order to verify the calculation method, we have carried out vibration experiment using a 1/20 scale model ship, and the experimental results were compared with the theoretical calculation on the 1/20 scale model, and reasonable agreement has been confirmed. Then, using the calculation method, we have estimated actual ship's natural frequency and vibration response, and studied a possibility of resonance of the tank system vibration with blade frequency and a relation between the vibration acceleration of tank and blade frequency.By these studies, it has become possible to determine the number of propeller blade.