Cylindrical oil storage tanks have been designed conventionally based on the brief calculation of elastic stress distribution on the components, because the shape of the tank is simple enough and the major part of hydrostatic load is supported by the foundation of the tank. Recently, some accidents of failure or damage have occured. Therefore, it has become necessary to re-investigate the safety against failure of the tank. Stress and deformation of cylindrical tank have been analyzed numerically with the consideration of the behavior of the foundation, and it has been found that the complicated behavior of deformation caused by the local sinking of the foundation can be explained by the results of the analyses. This may be useful for the structural design. On the other hand, from the standpoint of welding or maintenance of the tanks, it is necessary to examine and to analyze the part, where is most probable to cause failure, with the consideration of material property, geometric discontinuity and fracture mode. In this study, the model of Tee-joint which simulates the joint of cylindrical shell plate and annular bottom plate was examined and analyzed to research the deformation behavior of the joint and to investigate the criteria for design, welding and maintenance. As a result, it was found that the strain at the toe of weld on the annular plate, where fracture usually initiates under the bending load, is directly related to the change of angle between the shell plate and the annular plate. This relation does not depend on the material properties but on the plate thickness and loading condition. From the relation obtained above, by setting the allowable strain for the toe of the weld, the critical deformation angle of Tee-joint is successfully estimated, which can be adopted. as a design standard or a maintenance standard of this type of tanks. Finally, the requirements for bending ability and Charpy impact energy of Tee-joint are determined in order to prevent from brittle fracture. Futhermore, the completely penetrated groove welding of Tee-joint was compared with the fillet welding by means of experiment and numerical analysis. As a result, no difference was found between the both joints in regard to deformation ability and fracture stress. The incomplete penetration weld part which exists in the fillet welding joint has no influence on deformation and fracture behavior under the bending load. From this result, the conventional fillet welding may be satisfactory for the Tee-joint of the tanks.