Dependence of notch strength ratio on stress triaxiality and strain hardening exponent was investigated for a variety of structural steels. Both the notched specimens with different notch root radii and the unnotched specimens were employed to obtain a wide range of stress triaxiality. All specimens were loaded up to break under tensile loading at room temperature. Notch strength ratio was defined by a ratio of notch tensile strength to tensile strength of the material. The notch tensile strength required to break the notched specimen depends on the stress-state characterized by stress triaxiality. Notch strength ratio increased with increasing stress triaxiality, which eventually reached the limiting value in the range of high stress triaxiality. The limiting notch strength ratio showed a clear dependence on the strain hardening exponent, and could be used conveniently to estimate the upper limit load to break the mechanical component in failure analysis.%機械・構造物の破壊事故の原因究明に際しては，破rn断面の検査，金属組織的な検査など，多くの観点からrn検査を行い，事故原因を明確に特定する（1）．しかし，rn事故原因をより厳密に特定するには，破断時に加わっrnた負荷荷重の推定が必要となる．この場合，有限要素rn法，破壊力学に基づぃた解析，再現的な強度試験など，rn詳細な力学的検討が必要となる．
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