In this study, the characteristics was investigated of brittle crack propagation in mild steel. And it was experimentally obtained as the material property in relation to the crack velocity and the temperature and the dynamic fracture toughness K_D of common hull steel. Next, it was tried to forecast the crack propagation behavior in plates and beams structure. Using the double tension type large size specimen with stiffener, simplified procedure by modifying the L. B. Freund's solution was experimentally verified to be useful to predict the brittle crack velocity fluctuations. The procedure is based on the theory of the stationary crack propagation, and extended to the non-stationary crack. So when the boundary load makes no displacements during the crack propagation, that is, no additional works this method is seemed to be considerably promising.