Abstract: To study the propagating behavior of a dynamic crack in brittle material, an experimental technique was developed to measure the propagation speed of a fast crack in a preloaded brittle polymethylmethacrylate (PMMA) strip. The experimental results show that for each preloaded strip, the crack arrives at a steady velocity v₀ after a short acceleration stage, when the crack propagation is self-similar. The steady propagation velocity was found to be an increasing function of the energy G_c stored in the preloaded strip, which means that the material has a "speed toughening" property. When the crack speed exceeds a threshold, the crack speed exhibits apparent oscillations. This crack speed oscillation corresponds to the microscopic periodic grooves on the fractured surface. Further increase of the pre-stored elastic energy results in the curving, micro-branching, and full bifurcations of the cracks. Based on the energy conservation theory, a dynamic model is established to describe the motion of the crack. This motion equation is used to explain the crack speed oscillations during propagations.