In order to predict the fatigue life under two-stage loading by formulating a cumulative fatigue damage rule for composite materials, the fatigue process in SiC-Al and glass-fibre reinforced plastics were investigated. A microcrack occurred within the composites which resulted in cumulative fatigue damage that increased linearly with the number of cycles. The mechanical conditions of damage growth and failure were determined by characterizing the microdamage governing the fatigue. The ultimate failure is shown to occur when the product of the stress amplitude ratio and microdamage density is beyond a critical value and an expression for the remaining fatigue life is derived.