In the previous paper, the run/arrest events of a brittle crack relied upon the mechanical discontinuities around the pre-existing surface crack in uniformly stressed plate at a constant temperature were confirmed by using the modified double tension test technique. And the crack arrest properties of low tough 9% nickel steel used and its weldment were determined. In this investigation, the modified double tension tests were further conducted mainly on the low (P, S) type 9% nickel steel plates of 32, 23 and 16 mm thickness their weldments. As a result, the following conclusions were reached ; (1) The critical stress values at which a brittle crack could be arrested at the end of saw cut notch simulated the pre-existing surface crack significantly increased with decreasing of plate thickness. (2) Such effect of plate thickness is directing into the mechanism of crack arrest depend ing on the mechanical discontinuities. (3) This fact indicates that the thickness-independent crack arrest toughness expressed in term of K_Ia as a material constant does never exist as to the run/arrest event of a brittle crack. (4) The high toughness derived from the decrease in P, S content in steel brought a fairly increase in crack arrest properties, K_a of 9% nickel steels. (5) Further improvement of crack arrest toughness was confirmed on the direct quenched and tempered 9% nickel steel. (6) The crack arrest toughnesses K_a of low (P, S) type 9% nickel steel used and their weldments of 32 mm thickness were 514, 317 (fusion line) and 323 MPa√m (matching ferritic weld metal) at -170°C respectively.