The deformation mechanism of the fine structure composed of primary α phase (αp) and acicular secondary α phase (αs) on quasi-static mechanical properties is still not very clear.The main controversy is focused on the role of αp in the mechanical behavior.In this paper,the microstructure of the heat-treated near β-type Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy after tensile tests was observed by transmission electron microscopy (TEM).And the results showed that in the slight deformation region the dislocations were accumulated at the intersection of αp and β matrix separated by αs,while only a few dislocations nucleated in β matrix.In the severe deformation region,a large quantity of dislocations in both αp and β matrix were observed.It can be inferred that αp deformed firstly and then activated the deformation of β matrix,that is,the thickness of αp and the inter-particle spacing of αs played a dominant role in the deformation process.The quantitative relationship between the yield strength and the microstructure parameters is consistent with this inference.By adjusting the solution treatment param-eters and the subsequent aging treatment,three fine structures were obtained,and the corresponding mechanical properties were determined.Furthermore,the yield strength can be described by the mathematical model σy 756.4 + 135.6/hp1/2/2 +32.2/d1s/2,where hp and ds are the thickness of αp and the inter-particle spacing of αs,respectively.