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Uniaxial tension fatigue tests of (α+β) Ti-6Al-AV were carried out in vacuum and air environments to clarift how these environments affect the surface and the interior originating type fatigue properties. Then, the effects of the envinments on both fracture surface of stage 2a region and the condition of transition from stage 2a to stage 2b region were also investigated through fractographic analysis. As a result, the following conclusions were obtained : (1)The vacuum environment improved the fatigue lives of surface originating fractures although it gave no effect on those of interior originating fracture. (2)Crack propagation mechanism of surface fractures in the vacuum was different from that in the air. (3)The effect of the internal environment of materials on interior crack propagation was similar to that of the vacuum environment on surface crack propagation. (4)S-N properties were determined by the competition of surface fracture mode in high stress and interior fracture mode in low stress regardless of the environments.