The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon superbainitic steel were investigated.Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite,which will obviously enhance the tensile strength and the product of tensile strength and ductility.The higher the bainitic transformation temperature,the lower the transformation rate of the retained austenite.The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation.