It has been found that a sharp crack is formed on releasing the compression load applied to in excess of a certain value at a low temperature as liquid nitrogen temperature (-196°C), and it has been verified that this precompression cracking phenomenon can apply as a notch tip preparation method to fracture toughness testing. In this report, the suitable notch tip shape and proper preload magnitude for precompression cracking in specimens for fracture toughness testing are examined. Concerning the notch tip shape, 0.1 mm notch tip radius and 3060° opening angle are suitable for V shaped notch tip, and 0.2 mm width and 2 mm length are suitable for narrow width saw cut notch tip. Concerning the preload magnitude, the simple estimation method of precompression load in order to obtain a needed crack length and the determination method of proper pretension load are proposed. The precompression crack extension characteristics in welded joints are also examined experimentally using electrogas welded joint of SM 41 A, submerged arc welded joint of SM 50 B and shielded metal arc welded joint of HT 80. Cracks with a front sufficiently straight in shape are obtained in all joints by the precompression cracking method, although the crack extension length of HT 80/SMAW joint is changed largely in thickness direction by the fatigue cracking method. The precompression induced crack extension length can be estimated roughly by the proposed simple method.