Behavior of fast fracture and crack arrest is a matter of renewed concern today to structural engineering and many research works are being conducted. In the previous report, a dynamic fracture mechanics analysis on polymethylmethacrylate using finite difference method revealed that dynamic fracture of the material could be defined as a function of crack velocity. In this paper, dynamic fracture mechanics analysis is made on the experimental investigation using a ship steel to clarify the fundamental aspects of brittle crack propagation and arrest in structural steels. It has been found that dynamic consideration is indispensable, in general, for the interpretation of fast fracture and crack arrest and material property can be defined as a function of temperature and crack velocity. Using the dynamic fracture toughness obtained by dynamic fracture mechanics analysis on three types of brittle fracture propagation-arrest test, prediction of crack propagating behavior is successfully made for two extreme cases, i. e. no recovery and almost full recovery of kinetic energy to drive crack propagation. This approach is an elementary method of crack arrester design, and more relevant and practical crack arrester design procedure can be expected to be established by refining and modificating this approach.