The influences of second phase precipitations of low Young's modulus (E) β－Ti alloys [MoTi₁₄]Zr₁ (Ti_78.2Mo_11.2Zr_10.6) and [SnTi₁₄]Mo₁(Ti_75.7Mo_10.9Sn_13.4) on mechanical properties as well as the variation of phase constitutions of alloys during tensile test were investigated. Alloy rods with 6 mm diameter were prepared by copper－mould suction－cast method. Phase precipitations and microstructures of alloys before and after tension were analyzed by XRD and TEM techniques. The experimental results showed that only a minor amount of   α〞martensite precipitated on β matrix makes the suction－cast [SnTi₁₄]Mo₁ alloy possess lower E with a value of 70 GPa, while ω precipitation increases the E (80 GPa) of [MoTi₁₄]Zr₁ alloy. It is due to the thinner β plate twins that makes [SnTi₁₄]Mo₁ alloy have better mechanical properties. During tensile testing, the amount of  α〞martensite induced by stress increases in [MoTi₁₄]Zr₁ alloy while β grains in [SnTi₁₄]Mo₁ alloy are easier to be rotated for deformation, which favors to enhance the ductility of alloys.