To prevent the catastrophic failure of a large storage tank, it is necessary to ensure not only that the long brittle crack will be arrested by the material toughness, but also that the arrested crack will subsequently be stable. After the long and fast running crack is arrested, the bulging force applied to the crack flanks will grow and re-initiate the unstable ductile crack. R -curve concept combined with J -integral is adopted as a theoretical tool and wide plate tests are carried out to estimate the fracture resistance of a material for LPG storage tanks. The tested material is the 34 mm thickness 2 1/2% Ni steel plate manufactured by the special thermomechanical process, SHT-DAC. The specimens are 3 m in width and 4. 6 m in length so as to reproduce long arrested cracks in storage tanks. The specimens were loaded in the 10, 000 tonf test rig under the room temperature and as the ductile crack extended, they were unloaded in less than 10% for measuring the crack growth. The applied driving force is calculated from J -integral, the Battelle's equation on the bulging effect. The ductile resistance of the test material is 200 N/mm² as dR/da for long arrested cracks. For a large LPG storage tank of 40 m in diameter, the strake height should be limited under 3. 2 m to avoid the ductile instability of the long arrested crack.