The effects of oxygen content on microstructures, elastic modulus and tensile properties of quenched Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. From the microstructural and phase analysis, it is evidently revealed that the volume fraction of β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in metastable β Ti-Nb based alloys. Martensite transformation temperature decreased with increasing content of oxygen. Therefore, it is suggested that oxygen acts to stabilize β phase rather than α stabilizer in quenched state. Yield strength increased with increasing content of oxygen without a large consumption of ductility in metastable β Ti-Nb based alloys. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.