Abstract: For ultra-high strength steel and aluminum alloy, a fatigue crack could initiate from a notch tip under a cyclic compressive load. The th-resholp value Δσ_th or ΔK_th(ρ) for fatigue crack initiation under a compressive load was four times as high as that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing under cyclic compressive load and the maximal length of the fatigue crack was only 0.2-0.5mm. The crack nucleation under comdressive stress became easier and the propagation distance of the fatigue crack was longer if the mimimum cyclic compressive load was near zero.