Safety designs and fracture designs are the two important fields of crack theory. The fracture problems of middle carbon steel under extra-low cycle rotating bending fatigue loading are studied in this paper. The experiments have been made in special rotating bending fatigue fracture machine designed by us. Several problems, the relations of the cycle times of fracture and the strain amplitude near notch tip to the deflection, the effects of depth and tip radius of notch to the cycle times of fracture, are discussed through the experiments. The mathematical expression of fracture toughness to the cycle times of fracture is obtained. The suitable parameters of rotating bending fatigue fracture model for middle-carbon steel in extra-low cycle times has been proposed.