We studied the microstructure and impact toughness of low-alloy high-carbon steel austempered at low temperature. Ultrafine carbide-free bainitic microstructure was obtained, which is composed of lath-like bainitic ferrite and retained austenite. The impact absorbed energy of austempered samples is much greater than that of the low-temperature tempered sample, while the hardness is slightly lower than the tempered sample. Crack propagation occurs via a combination of brittle quasi-cleavage fracture and ductile dimple fracture.