Crack propagation in bcc iron at different strains under low temperature (30K) has been studied using the atomistic simulation. We show that cracks display a brittle character of extension at low strains, and at relative higher strains cracks extend with a periodic series of twins(or SF) bursts. These bursts decrease the crack speed and produce velocity oscillations with an increase in energy dissipation that increases the toughness. Here we also develop a new form of dynamic fracture energy. Using our form of dynamic fracture energy, the results therefore are in quantitative agreement with the theoretical single-crack equation of motion.