At present, the most reliable fatigue design for structures subjected to random loading, such as ships and offshore installations, is considered to be one based on fatigue strength diagrams, which would result form actual experiments with random loading. However it would take a very long time to complete the necessary experiments, so long as to be impossible. The authors propose a new method of developing fatigue strength diagrams for crack initiation and propagation life. This method combines random loading fatigue tests plus an equation containing a novel assumption, which has been shown to be useful in estimating fatigue strength under random loading condition. In this investigation, using the proposed method, mild steel center notched specimens were chosen for developing crack growth rate diagrams. The frequency distribution of stress levels is exponential in distribution. Tests were performed under load controlled axial pulsating tension. In addition, a crack opening point and a threshold stress intensity factor were examined.