Elastic modulus and mechanical property of Ti-Nb-O alloys prepared by arc melting and subsequent water quenching were investigated in order to correlate the result of microstructural observations and phase stability. In quenched state, it was evidently observed that the volume fraction of bcc-structured β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in the present Ti-Nb based alloys. This microstructural result was supported by changes in martensite transformation temperature, which decreased with increasing content of oxygen within the chemical composition range investigated. Therefore, it is suggested that oxygen plays a role as a β phase stabilizer rather than α stabilizer in metastable β Ti-Nb alloys. Yield strength increased with increasing content of oxygen without a large consumption of ductility in Ti-Nb based alloys with Nb content of 24% while elongation value decreased with increasing content of oxygen in Ti-Nb based alloys with Nb content of 28%. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.