Fracture toughness defined near the initiation of stable crack growth is investigated by R-curve and Direct Current electric Potential Determination(DCPD) under modeⅠplane strain conditions for CT specimen with 25.4㎜ thickness of SS400 steel. Fracture toughness, near crack tip of CT specimen by R-curve is 17.14 and however, its value by DCPD is 22.82. The value of fracture toughness by DCPD is larger than that by R-curve. Therefore, it is suggested that the evaluation of fracture toughness by R-curve is optimum than by DCPD, when considering amount of crack growth about each of fracture toughness.