The pseudopotential method and the generalized gradient approximation have been employed to calculate the elastic constant, cohesive energy and electronic structure of β, α″ and ω metastable phases in the Ti-25at%Nb alloy. The phase stability of β, α″ and ω phases is discussed adopting these calculated results. The results show that all of the β, α″ and ω phases satisfy the criteria of elastic stability. The phase stability of α″ phase is the highest and that of β phase the lowest among β, α″ and ω phases. The results further show that the Young’s modulus of ω phase is the highest and that of β phase the lowest in the Ti-25at%Nb alloy.