As the authors showed in a previous paper, the criterion for obtaining plane strain at the mid plane of the specimens is established as:β≥0.004<(Κc(J)/σys)>2+0.01. However, it is not clear whether this equation can be applied in the region of low transition temperature or in the low shelf region. Therefore, fracture toughness tests were performed in this temperature region to check the validity of the above equation, regardless of temperature, using a mild steel. In addition, the influences of the curved precrack shape on stress state and crack initiation sites near the crack tip were investigated. No difference was recognized between the straight crack and curved precrack shape in the distribution of fracture toughness and crack initiation sites. The results showed that the above equation is valid in the region of low transition temperature but invalid in the low shelf region.