Engine and turbine components that experience high cycle fatigue should operate under the fatigue threshold stress level to ensure their safe service life. In general, designers have to consider two types of thresholds, i.e. fatigue stress threshold for crack initiation, and fatigue stress intensity factor threshold for crack propagation. These two thresholds overlap for short cracks or materials with defects. A new approach, which is called the fatigue endurance diagram (FED) for the prediction of fatigue threshold of materials with defects, is presented. This approach indicates that the fatigue strength of materials with cracks or defects is determined simultaneously by two parameters, the normalized stress intensity factor range and the normalized stress range. The FED indicates that the fatigue threshold condition is governed mainly by the threshold stress intensity factor and by the fatigue limit for the long and short cracks, respectively.