In this report, the fatigue properties of an ausformed 12-Cr stainless steel have been clarified by means of S-N curves, crack growth rates and microscopic observations of propagating cracks and fractured surfaces.The results obtained are as follows:(1) The endurance limits of a plane specimen are improved in proportion to the reduction ratio of ausforming and the fatigue lives at a definite stress level are lengthened markedly by ausforming. At the same time, the crack growth rate slows down in the ausformed specimen.(2) The modes of crack growth, precipitation of carbides and fractographs differ clearly between conventional and ausforming processes and these differences seem to cause the above results.(3) The crack growth rates are represented by the equations which contain a stress intensity factor and a parameter of mean stress in each process. By these equations, we can recognize that the effect of mean stress on crack growth rate becomes larger for the ausformed specimen.(4) The fractographs show that the fracture surface of the conventional heat treated specimens exhibit either relatively rough striations or intergranular fracture patterns, while the fracture surfaces of the ausformed specimens are almost covered by the mixed patterns of rather irregular striations and flat areas which look like rub marks.