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Using finite element method, modified boundary layer model for homogeneous and heterogeneous welded joints are analyzed. After investigating the relationship of the opening stress at the crack tip in these two cases, a new constraint parameter is developed for overmatched welded joints, allowing the material mismatching effect on the crack tip stress to be quantified. The new constraint parameter is based on the 2JA− three-term solution which is a reasonably good approach to describe the geometry constraint at the crack tip. In the case of complete specimens, constraint arising from both geometry and material mismatching exists. The total constraint has been obtained in establishing the relationship between 2A and2mAwhich is a new constraint parameter related to the mismatch. Finally, using the experimentally determined fracture toughness at one mismatch level, we predict the fracture toughness values at other mismatch levels. The result is consistent with the experiment data.