Although a large variety of analytical and experimental studies have been done of fracture behaviors of specimens with through-thickness crack, critical assessment in actual structure is strongly required for part-through-thickness defects. For engineering purpose, presently, part-through-thickness cracks have been dealt with by assuming that the parameter governing flaw shape effects would be the same those under linear elastic conditions. In the present paper, experimental and analytical considerations have been conducted whether or not the engineering procedure becomes a safe assessment of welded defect. For the specimens with a part-through-surface-notch, K _c -concept according to LEFM can be applied for small scale yielding fracture. For general yielding fracture, δ _c -criterion according to the definition of our paper can be effective. The assessment procedure based on the equivalent through-thickness crack parameter a_K obtained from LEFM analysis should be applicable even for general yielding fracture and for specimen under plate bending. Therefore, BSI and WES approaches proposed already using the above assessment procedure become almost conservative from the standpoint of critical assessment for the unstable fracture of welded joint with a part-through-thickness defect.