This paper describes the crack arrest properties of 9% nickel steel and its weldment for LNG service applications. Supposing the situations where the surface crack exists in the plate being uniformly applied a constant stress and kept at a constant temperature, a possibility of restricting the potentially extensive propagation of a brittle crack was discussed. The magnitude of stress intensity factor in the vicinity of the pre-existing surface crack shows the maximum value at the deepest crack tip located in the middle of plate thickness where the plane strain condition is maintained, and shows the minimum one at the both ends of surface crack in the plate surface where are in the plane stress condition. Therefore, there remains possibilities of crack arrest in the end of surface crack even if a brittle crack initiated and propagated in the bottom region of surface crack. In this investigation, such run/arrest event was confirmed in the modified double tension test. This special testing technique is characterized by the specimen with the sharp notches simulating the pre-existing surface crack. The sharp notches have an adequate length and are located in the crack propagation part of specimen. The crack arrest properties, K_a as the material constant were evaluated by the correlation of applied stress and arrest crack length obtained in the modified double tension test. The dimensionless stress intensity coefficient Y after Srawley's numerical analysis on single-edge crack model was used in the calculation of K_a . As a result, it was determined that the low tough 9% nickel steel used ( _vE _-196=84 J) and its weldment had superior crack arrest properties of 220 and 259 MPa √m at -170°C respectively.