It is generally recognized that the fatigue failure of the cracked engineering components is a crucial issue, especially considering the sample thickness effect. Although numerous methods have been proposed, it is still unclear about the sample thickness effect on the fatigue crack growth (FCG) rate of metallic materials. Based on the plasticity-induce crack closure and crack tunneling effect, we established an improved FCG model with a thickness factor T by the load analysis near the crack tip. The thickness factor could clarify the sample thickness effect on the FCG rate of high-strength steels. Numerous fatigue data demonstrates that the improved model displays a fairly good relationship between the sample thickness and FCG rate. It would provide a new strategy to the FCG rate prediction of cracked structures through small size specimen.