Abstract: The procedure of the crack acceleration and deceleration in a cantilever beam is seized with the help of high-speed photography, and the dynamic stress intensity factors near the crack tip are determined by dynamic caustic patterns. By means of the data processing of least square filing both the horizontal and vertical displacement components, the tip positions and the curving directions of the crack are obtained, which show the history of the acceleration of the crack moving. The results indicate the dependent relations between dynamic stress intensity factors and crack accelerations.