Main purpose of this investigation is to estimate fatigue life or allowable stress under random loading without using well known cumulative damage laws. Basis of the analysis is to obtain fatigue strength diagrams, namely S-N curves, under random loading, and to determine fatigue life or allowable stress directly from the above S-N curves. Therefore, it is necessary to develop reasonable fatigue test method under random loading and to get S-N curves for random loading experimentally or analytically. For this reason, it is required to study statistical characteristics of fatigue life under random loading and to compare with results of constant loading tests under various conditions, for example, kind of materials ; type of loading ; shape of specimen or model structure ; range of fatigue life, etc. To achieve the above purpose, the following experiments were carried out and obtained statistical values of fatigue life and cumulative damage sum by Miner's Rule. The experiments are axial reversed fatigue tests with about 30 pieces of mild steel plain specimen in life range of about 3×10⁴ cycles, and loading conditions are ( a ) constant loading amplitude, and ( b ) load and strain controlled random loadings with exponential distribution. From results of these tests, the following conclusions were obtained. (1) It was found that probability distribution of fatigue life under the above loading conditions conformed with normal or logarithmic normal distribution approximately, by plotting tests on the probability paper and X ² tests. (2) Using F tests, it was clarified that scatter band of fatigue life under random loading was considerably narrower than that of constant loading and the band of the life under strain controlled random loading was little smaller than that of load controlled random loading. (3) Probability distribution of cumulative damage sum is nearly equal to normal distribution. (4) Cumulative damage sum is less than 1 and arithmetic mean value of the sum is O.222 for load controlled loading tests and 0.551 for strain controlled ones.