Generally, there may be possibility that brittle fracture would start from a welded joint of welded steel structures, because of some weld errors such as angular distortion and misalignment, some defects such as weld cracking or fatigue crack, and low toughness of welded fusion line. In order to prevent structures from brittle fracture, it is necessary to have enough knowledge about brittle fracture initiating property of welded fusion line which is superposed some weld errors and defects, and to know the toughness which is required for welded steel structures. In this study, brittle fracture test of so-called large scale tensile specimens with angular distortion, misalignment and surface notch was conducted with 8000 ton and 2000 ton test rigs using weld joints of 25, 50 and 75 mm thick mild steel, 50, 60 and 80 kg/mm² tensile strength steels. The surface crack of this type specimen is exposed under the superimposed. condition of tensile stress and bending stress. A general approximate formula of stress intensity factor for such a semi-elliptical surface crack was derived from G. R. Irwin's solution and A. S. Kobayashi's numerical results. This formula was proven to explain the results of simple tensile and pure bending tests. Furthermore, the bending stress behavior of the tensile specimen with angular distortion and misalignment was analyzed, and a general solution of bending stress for elastic end condition was obtained. Using these formulae, critical stress intensity factor for brittle fracture of such a specimen that was tested in this study can be evaluated reasonably. In order to evaluate brittle fracture of steel structures with a quality control test, the relation between the critical stress intensity factor of the large scale test and the absorbed energy of Charpy test was investigated. Then, modifing Y. Hagiwara's correlation formula between critical COD and Charpy impact energy, a statistical correlation formula was obtained. As a result, it is possible to assess welded steel structures against brittle fracture initiation with fracture mechanics approach using the result of Charpy test which is one of quality control tests.