In order to seek a more reasonable dynamic theory for fast fracture and crack arrest, the authors carried out a dynamic fracture mechanics analysis with the use of finite difference method and revealed that the method is a useful tool to analyze the crack propagation and arrest as reported in the previous paper. This report describes the results of an experimental investigation using polymethylmethacrylate (PMMA) and is a preliminary report on the study of dynamic aspects of brittle crack propagation and arrest in structural steels. Using dynamically measured data in the experiment as the boundary condition for finite difference method, mechanism of fast crack propagation and arrest in PMMA was analyzed by energetics considerations. As one of the most interesting results, a particular relation between dynamic fracture toughness and crack velocity was obtained, i. e. crack acceleration and crack deceleration followed two different curves and the crack arrest stress intensity factor was found to be substantially lower than the critical stress intensity factor for crack initiation. Further studies on the crack arrest phenomenon in structural steels using the procedure described in this paper should be conducted in the near future.