Abstract: The isothermal transformation kinetics of experimental steel was studied by the thermal expansion method. Uhrafine bainite was obtained by austempering at low temperature. The microstructure was quantificationally analyzed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that microstructure of the experimental steel is composed of a large number of uhrafine lath bainite ferrite and thin film austenite. The spacing of the bainitic lath is about 60 nm and the hardness is about HBW 610 by austempering at 210%. The final microstructure and properties depend on the temperature and holding time for austempering. The holding time for fully transformed bainite increases at a lower austempering temperature.