Brittle fracture in carbon steel seriously impacts structural safety. It is considered that the elementary step of the brittle fracture of polycrystalline steel corresponds to cleavage in each crystal grain and their connection process. However, the detailed mechanisms of brittle fracture are not completely understood. In this study, the elementary process of brittle crack propagation is clarified using the dynamic strain recording of a strain gauge near the crack path.The results indicate that the brittle crack propagation rate in a single crystal grain is much slower than the Rayleigh wave rate. To estimate the dissipation energy during crack propagation in a single crystal grain, dynamic finite element analyses were conducted by assuming constant critical stress during crack propagation. The dissipated energy is not small even inside a single crystal grain and appears to exhibit a proportional relationship with the stress intensity factor.