This study evaluated the influence of the strength mismatch of HAZ for a plastic ${eta}$-factor, which is the principle parameter determining the plastic portion of J-integral to assess the fracture toughness of the weldment. The specimen of tensile and hardness test was manufactured from the piping applying narrow-gap welding, and the mechanical properties of weldment, HAZ and a base metal were obtained. To perform the finite element analysis according to the ratio of strength mismatch, the material properties was chosen with the change of strength using the determination method of Ramberg-Osgood constant. Also, the influence of the strength mismatch of HAZ was determined using finite element analysis by those properties.