The microstructures and mechanical properties of an ultrahigh-carbon steel with 1.37 wt.%C subjected to austenitization at single γ phase region and austempering at low temperatures have been studied. The results show that a superbainite comprising lamar bainite ferrite and film-type retained austenite is generated. An impact absorbed energy of 12.7 J,a fraction strength of 1 622 MPa and an elongation of 5.5% are obtained for UHCS-1.37C austempered at 310℃ for 8 h. The combination of excellent strength and toughness is attributed to the superbainite.