This paper describes the effects of stress ratio on the ΔK_th improvment by overload. Tensile overload was applied to compact tension specimen, subsequently fatigue tests were carried out at stress ratio S= 0.1 or 0.5. And an analytical investigation of effect of stress ratio was studied. The value of ΔK_th increased with increasing of the tensile overload . And the nominal threshold values were given by the equation: Δ^NK_th^s=C + DK_ov. Where C is ΔK_th^o and D is a proportional constant and 0.26-0.28. Experimental results showed that D showed good agreement with theory. Thus, if Δ^NK_th^s is enough larger than applied K and ΔK values, crack is not able to grow and can be made harmless.