As showed in the previous paper by authors, for the estimation of fatigue life on allowable stress for fatigue failure of structural members and mechanical components, etc., the method based on S-N diagrams obtained from random loading fatigue tests is considered to be much more reliable than the method using cumulative damage laws. But it will take a very long period of time to carry out the above tests completely, and is considered to be impossible actually. Therefore, unavoidably Miner's Rule is generally used at present. In this paper, a new method of fatigue test was proposed. It is a combined method of random loading fatigue tests and analytical calculation, and is able to obtain the complete S-N diagram under random loading during a very short period of time. To inquire the applicability and the accuracy of the proposed method, the following random loading fatigue tests were carried out for plane and notched mild steel specimens. They are axial reversed load controlled tests with exponential distribution. Estimated S-N diagrams in the region of under fatigue limit were compared with experimental results and a very good agreement was found quantitatively. So the proposed method is considered to be very useful for the estimation of fatigue life or allowable stress under random loading.