The mechanical properties and tensile deformation mechanism of Ti-6.5Al-3.5Mo-1.5Zr-0.25Si alloy (TC11) with different percentages of primary α phase (αp) were studied by conventional and interrupted tensile tests, respectively. The results show that the strength of TC11 alloy increases with the increase of the percentage of αp. During tensile deformation, TC11 alloy with different percentages of αp exhibits similar behavior. The deformation mechanism in αp transforms from single slip to multiple slip. While, the microcracks first initiate at the interface of two phases, propagate along it, and then propagate through the transformed β phase (βt) and finally through αp. Finally, a method for predicting the strength of TC11 alloy considering the sizes of αp and βt is proposed.