It is known that the remarkable effect of the pre-existing defect on the brittle fracture initiation characteristics of welded joint is mainly caused by the interaction between the residual stress and the deterioration of the fracture toughness due to the weld thermal strain cycles at the tip of the defect, so called the hot straining embrittlement. This embrittlement is not always accompanied with the change of metallurgical structures and moreover the region of the embrittlement is limited only in the very vicinity of the tip of the defect. Nevertheless it is clear that this embrittlement is one of the most important factors which cause the low stress brittle fracture initiation even in the service temperature in the case of the mild steel and 50 kg/mm²class high strength steel. In the present paper, various factors which have considerable effects on the hot straining embrittlement are extracted and effect of the temperature and the amount of the strain at the tip of the defect on the deterioration of fracture toughness are quantitatively evaluated. A simple testing method which uses a small size COD specimen prestrained at 250°C is developed to evaluate the susceptibility to the hot straining embrittlement. A mechanism of the hot straining embrittlement is studied by the results of this testing method, so called PBCT (Pre-compression Bending COD Test).