The welding material for 9 % Ni steel is usually face centered cubic Ni-bace alloy, the yield strength and Young's modulus being considerably lower than those of steels. Welded joints of 9 Ni steel are usually soft welded joints ; the stress for the brittle fracture initiation of the joints, whose HAZ has a crack, is influenced distinctly by the mechanical properties, aside from the metallurgical properties. The linear fracture mechanics, therefore, can not be applied to btittle fracture initiation of the soft welded joint. A modified BCS model was introduced in this paper ; the COD of a crack in the vicinity of the soft weld metal was expressed quantitatively by using this model. Deep notch tests of the HAZ in 9 % Ni steel welded joints were carried out by using specimens which have several notch lengths. The application of the modified BCS model to the experimental results showed that COD-concept could be applied to the brittle fracture initiation of the soft welded joint. Therefore, size effects on the brittle fracture initiation of the soft welded joint was expressed quantitatively. The experimental results also showed that the fracture stress of base metal which has a crack in the vicinity of the soft welded joint is lower than that of base metal free from the effect of the welding. These facts were also explained quantitatively by the modified BCS model.