Abstract: The effect of Mo on the microstructure and property of fire-resistant steel is studied by means of dilatometer and transmission electron microscopy. It is found that the chief effect of Mo in fire-resistant steel is to promote bainite transformation to ensure the high temperature properties, therefore, in view of microstruc-tural controlling, it is possible to reduce Mo content by increasing the content of other elements which can enhance bainite transformation in the design of chemical compositions of fire-resistant steel. The feasibility of designing Mo-reduced and Mo-free fire-resistant steels is discussed.