The basic premise of the MOSS-Rosenberg LNG containment system and other spherical LNG tank system is “leak-before-failure”. In other words, the design concept applied to this type of pressure vessel is basically “fail-safe”. Therefore, to evaluate the structural safety of the spherical LNG tank, it is neccesary to obtain the fracture toughness of the tank shell plate and the welded joints, quantitatively. Considering the strength of brittle fracture initiation of 9% Ni steel LNG tank, the fracture toughness in the heat affected zone is the most important problem. Accordingly, the authors carried out the tension test with wide center-notched plate test specimens, notched in the heat affected zone of submerged arc welding with heat input values of about 1050 KJ/cm, in order to evaluate the fracture toughness of welded joints of 9% Ni steel. The following conclusions were obtained. (1) It is reasonable to calculate the critical crack length, using the experimental results obtained from the standard size specimens (about 400 mm wide) on the basis of Linear Elastic Fracture Mechanics. (2) For the butt-welded joints produced with the most practical heat input values of about 1040 KJ/cm, the lowest K _c values at the cryogenic temperature of LNG, -62°C, will be 775500 kg, √mm/mm², and the critical crack length for brittle fracture initiation of 9% Ni steel LNG tank is satisfactorily long.