The objective of this paper is to investigate the relationship between fracture toughnesses of 9% Ni steel at brittle crack initiation and at brittle crack arrest. It is the authors' point of view that the difference between these fracture toughnesses is due to the difference in strain rates. Based on this assumption, the effect of strain rate on the fracture initiation toughness was investigated by three-point bend test, while the fracture arrest toughness was investigated by Compact Crack Arrest (CCA) test. On comparing fracture initiation toughness, K _c , with fracture arrest toughness, K _ca , it was found that K _ca was almost equal to or higher than K _c . The cause is considered to be the effect of ductile lips left below the plate surfaces. This led the authors to the conclusion that an initiated brittle crack can inevitably be arrested after a short extension even if external force does not fall. To verify the finding, a notched very wide plate specimen was tested. The result showed that only a pop-in type of fracture could occur, that is, an initiated brittle crack was arrested after the extension comparable to plate thickness. The notched wide plate test also verified that the reinitiation of brittle fracture from the arrested crack could not take place without the substantial increase in external load.