Abstract: Ti-5Al-5Mo-5V-3Cr-1Zr alloy was treated by to aging at 580℃ for different times (0, 5, 15, 60 min), and then α phase precipitation behavior and its effect on the tensile fracture mechanism of the alloy were studied. The results show that α phases with triangle or V shaped, dendritic and cluster morphology successively precipitated in matrix β phase of test alloy with the increase of aging time. The orientation relationship between α phase with different morphology and β phase obeyed the Burgers rule. During the tensile deformation, the dislocations in the structure with triangular morphology α phase were concentrated in the large-sized strip α phase, and the cracks initiated along the preferentially deformed α phase. The dislocations in the structure with dendritic morphology α phase were concentrated in parallel strip α phase; the cracks initiated along the trunk α phase, and then the quasi-cleavage plane was formed. The dislocations in the structure with cluster morphology α phase were activated in β phase between strip α phase; the crack initiated along the high angle β/β phase interface, and then the smooth quasi-cleavage plane was formed.