In order to investigate the influence of strength mismatching on brittle fracture strength for large heat-input welded joints, a series of tension tests was conducted using surface notched specimens. Furthermore, notched round-bar tension tests were carried out for simulated bond material (homogeneous material) which was base metal heat-treated on the same condition as the weld bond. Thereby, cleavage fracture strength of the bond material without the influence of strength mismatching was obtained. The point of fracture initiation was identified by SEM observation and, the critical maximum principal stress at the point (σ1_c) was calculated by FE analysis for each specimen. σ1_c values of surface notched specimens were found to be distributed in the range of 900-1,400N/mm². This result indicates that the strength mismatching does not significantly affect σ1_c. In addition, distribution of σ1_c for the notched round-bar specimens is in good agreement with that for surface notched specimens. This indicates that the cleavage fracture of the weld bond is a dominant fracture on the welded joints. Peak values of maximum principal stress σ1^p and equivalent plastic strain ε_p^p increase with increase in the strength mismatching ratio defined as the ratio of yield strength of weld metal to that of HAZ (σ_Y^WM/σ_Y^HAZ). The increase in σ1^p causes the decrease in fracture strength of the weld joints. ε_p concentration at notch tip of HAZ side is due to the lower yield strength of HAZ. Except for small-scale and large-scale yielding conditions, the influence of ε_p distribution caused by strength mismatching on fracture strength also needs to be considered.