The high cycle fatigue (HCF) at high temperatures of bimodar size lamellar O pahse microstructure of Ti-22Al-25Nb alloy is studied. HCF strength limits of bimodar size lamellar O pahse microstructure were mearsured via a method named as small sampling up-and-down method (SSUDM). The HCF strength limits at 650℃ and 700℃ were measured to be 470MPa and 400MPa respectively with the stress ratio of -1 and the controlling cycle of 107. It is found that the samples with HCF cracks initiated at subsurfaces can provide significantly longer HCF life than the samples with HCF crack initiated at surfaces. It is found that transformation B2→β+O occurred in the mode of cellular precipitations at initial B2 grain boundaries during the HCF tests. The transformation resulted in inhomogeneous regions in the microstructure. The inhomogenieous regions are the prior cracking areas.