Hereunder is presented the report of the studies that the authors have made about the effect of mean stress on fatigue crack propagation rate at room temperature and at various other low temperatures. Some of authors' conclusions have appeared to be different from those of other investigators. To make clear these apparent inconsistencies the experimental data of others have been rearranged, assuming that the propagation of fatigue crack is devided into four stages, the initial, the tensile mode, the shear mode and the final propagation stage. As the result, it has been proved that there is no inconsistency after all between authors' conclusions and others'. Besides there has been new information that has been obtained in this connection.