The construction of large-sized membrane type LNG carrier needs several high level technologies, among them sloshing is one of the most important problems. LNG carriers of Gas/Transport system have achieved a great deal of successful service results, but some damage of tank top insulation (minor deformation of top plate of a few primary insulation box) by sloshing occurred on two ships, though filling ratio of tanks is limited over 90 pecent or empty. So, investigation for sloshing of high filling ratio with model tests, plywood box crash tests, and then, analysis of damage were carried out. And it is concluded that 1. High sloshing pressure occurs, even if the filling ratio is high. 2. If countermeasure shall be duly applied, it shall be easily overcome. And then design pressure is proposed for newly designed 130,00 cu m LNG carriers. In this paper, the investigations, analysis and proposed design pressure are summarized.