Fatigue tests have been conducted on the cruciform flat specimens of a weldable structural steel and a stainless steel with a center crack by using a high-cycle biaxial fatigue test facility, in order to investigate the effect of biaxial stress on the fatigue crack growth properties ΔKI-da/dN and the threshold conditions ΔKIth. From these comprehensive experiments, no effect of biaxial stress was found on both ΔKI-da/dN relation and ΔKIth, if the crack was large and the stress level was low. However, a significant effect of biaxial stress appeared when the crack was small and the stress level was high.The critical region where the effect of biaxial stress appears was clarified quantitatively, based on the crack length measured and the stress level used in the present biaxial fatigue tests as well as those of other experimental works. Moreover the elastic-plastic FEM analysis was carried out on a crack in a cruciform specimen under biaxial stresses. The effects of biaxial stress on the plastic region and the crack opening displacement at a crack tip were discussed in comparison with the effect on the fatigue crack growth.