This study would like to evaluate the influences with the strength mismatch and the variation of the welding width of the narrow gap welding for the plastic parameter, the major constant determining the plastic ${eta}$-factor of J-Integral, using 3-D FEM. For this, we evaluate the plastic ${eta}$-factor according to the variation of the strength mismatch of weldment with same materials and welding width through FEM. Also, we proposed the equation of plastic ${eta}$-factor considering the variation of the strength mismatch of weldment with similar materials and welding width.