Fatigue crack propagation behaviour under the condition of mixed modes I and II applying was experimentally studied by applying tensile and torsional fatigue loadings of the same phase simultaneously on thin-walled hollow cylindrical specimens. As the initial component of ΔKII increased, the fatigue crack growth rate accelerated in the low crack growth rate region. In the rigion, where da/dN=A(ΔK)m holds, that is, in the second region of da/dN versus ΔK curve, the ratio of the initial ΔKII to the initial ΔKI, ΔKII/ΔKI, increased and reached near the value of 0.58, the fatigue crack growth rate became much lower than that at any other condition. When the value of initial ratio ΔKII/ΔKI increased further more, for example, ΔKII/ΔKI reached 2.16, the fatigue crack growth rate accelerated again. Under shear fatigue loading, the crack growth rate was higher than that of tensile fatigue loading.