Small Butterfly valve is specially designed for use in industry field and ship. The important parts of butterfly valve are composed of disk, stem and body. But, like LNG ship, in enviroment of low temperature, as working condition deteriorates, important of sealing more increased. In this study, we examine sealing mechanism of butter valve on the base of FEM and investigate sealing life by measurement. The finite element analysis was carried out to study the effect of the seal ring shape on bi-directional sealing force of valve. The sealing mechanism was evaluated by 2-dimensional model in order to save the analysis time. And these analyses used by material non-linearity and contact element were implemented.