It is well known that Miner's cumulative damage sum at fracture is widely scattered and reason for the scatter of damage sum is not yet made clear. But, at present, Miner's rule is used in most cases to analyze fatigue strength of structural components under random loading conditions in spite of existence of uncertainty for application of the rule. The purpose of this investigation is to find the above reason and to get the suitable value for critical damage sum at failure. To achieve the above purpose, the following experiments were performed on plane specimens of mild steel in life range of about 3 × 10⁴ cycles. The experiments are axial reversed fatigue test with a minimum amplitude larger than fatigue limit, and loading conditions are constant loading amplitude, load and strain controlled random loading with exponential and uniform density. The relationship between Miner's cumulative damage sum and the stress-strain response under random loading was discussed. As a result, it became clear that there existed good correlation between the critical cumulative damage value and the following parameter (σ _YR ' k_R n_R ) / (σ _Yc ' k_c n_c ), where σ Y_c ' and σ _YR 'are yield stresses, and k_c , n_c , and k_R , n_R are coefficients in stress-strain response ( _a /σ _Y ) = k (ε _a /ε _Y ) ⁿ , S_a , ε _a : stress and strain amplitude, σ _Y , ε _Y : yield stress and strain, under constant amplitude and random amplitude loading respectively.