Abstract: There are many literatures on fatigue cumulative damage. Up till now, all kinds of cumulative damage theories considered the loading sequence point out that for specimen under high-low loading, the cumulative damage∑ni/Ni＜1. But these theories reflect only the one aspect. According to the stability of microstructure in metallic systems, this paper proposed that for cyclic hardening material, as long as the first high level load cycles. Are small enough, the cumulative damage ∑ni/Ni must be more than unity under high-two- level loading. And deduced that is also true under multi-level program load. But for cyclic softening material the result turned out contary Experiment under high-to-low two-level loading has testified the analysis for plain specimen of cyclic hardening material. On the basis of the experiment, some ideas about the estimation of fatigue cumulative damage are proposed. Key words; Fatigue cumulative damage, cyclic hardening, stability of microstructure in metallic systems, program load. But up to now, all Cumulative damage theory refleeting loading sequence belived that the Cumulative damage ∑ni/Ni＜1.Has to be occurred when specimen is with fatigue damage under multistage load in sepuence from high to low.