In the previous papers, the authors proposed a realistic crack arrest concept to assess the structural integrity of LNG storage tank and the surface-notched double tension test to evaluate the crack arrest properties of 9% Ni steels and their weldments. This paper deals with dynamic analysis of the run/arrest events confirmed in the surface-notched double tension tests conducted on 9% Ni steels by three-dimensional linear elastic finite element method. Prior to the analysis, dynamic deformation behaviors of the specimen and crack velocities were measured under the potentially extensive propagation and arrest of brittle crack during testing. These experiments verified that a brittle crack propagated through the specimen and/ or was arrested at the notch tip under the fixed grip conditions. These experimental data were used as input data in the analysis by general purpose program ADINA. Crack propagation was simulated in such manners that the nodal points on crack propagation line were constrained by truss elements which were eliminated at the time interval corresponding to crack velocity. Excellent agreement was obtained between experimental and FEM results as to the dynamic deformation behavior of specimen. Variation of dynamic stress intensity factor with crack propagation was obtained by energy balance theory. Dynamic analysis shows that K_D decreases as the notch depth becomes shallow and that K_D indicates minimum value at the notch end. In linear elastic fracture mechanics, the arrest of propagating crack is to occur when dynamic stress intensity factor K_D of the crack becomes lower than crack arrest toughness K_a of the material. Present results of experiments and analyses do not necessarily support the K_a concept stated above because it happened that K_D of the propagationg crack showed lower value than that of the arrested crack. Crack arrest at the notch end occurs if sufficient plastic deformation is allowed there when triaxiality at the crack tip is low and crack velocity is slower than plastic wave velocity. Since the surface defects existed in the welded structure are generally small, there hardly exists the possibility of crack propagation failure of 9% Ni steel and its weldment in LNG storage tank.