The crack speed and the dynamical stress around a propagating brittle crack were measured using the ESSO Brittle Test specimens. The plastic surface energy was obtained from the measured crack speed by applying the modified Griffith conception. The results obtained during this investigation can be summarized in the following conclusions : (1) The measured dynamical stress around the propagating crack front decreases with the increasing crack speed and fairly agrees with the theoretical one obtained in the previous report. (2) The crack speed increases with the increasing applied stress, plate thickness, and the decreasing temperature. (3) The plastic surface energy decreases with the decreasing temperature and the increasing plate thickness. (4) The plastic surface energy of quenched and tempered steel “2H” is much greater than that of normalized killed steel.