The measurements of fatigue crack propagation rate in 7075-T6 aluminum alloy and mild steel SS41 sheet specimens have been made to determine the effect of stress ratio on the propagation rate. The effect was clearly recognized in 7075-T6, but not in SS41. This observation was well explained in terms of the accumulated plastic strain rate, which is defined as the proportion of the accumulated plastic strain in the monotonic plastic zone to the nominal fracture plastic strain. The plastic strain in the monotonic plastic zone for Mode I of crack opening was calculated by analogy to the plastic strain distribution in Mode III given by Rice. In 7075-T6, the accumulated plastic strain rate clearly increased in proportion to the stress ratio, but in SS41, the rate hardly increased even if the stress ratio increased. It was concluded that the effect of stress ratio on propagation rate depends on the degree of increasing rate of the accumulated plastic strain rate corresponding to the increase in stress ratio, which in turn depends on the degree of the nominal fracture plastic strain. This conclusion has been substantiated through a survey of the correlation between the effect of stress ratio and the nominal fracture plastic strain on various materials reported heretofore. It was established that the effect of stress ratio is clearly recognized in the materials having nominal fracture plastic strains of less than 50% (i.e., the reduction of area is less than 33%), but none is recognized in the materials having nominal fracture plastic strains of more than 80% (i.e., the reduction of area is more than 44%). By considering the relation between the accumulated plastic strain rate and the material's constant c in fatigue crack propagation rate law (dl/dN=c(ΔK)m), it is shown that the effect of stress ratio can be estimated from the accumulated plastic strain rate quantitatively.