The composition of Ti-30Nb-10Ta-5Zr, which is simplified that of the Ti-29Nb-13Ta-4.6Zr alloy developed for biomedical applications, was selected, and then Nb content in the basic composition was varied from 20 to 35%. The deformation mechanisms of such Ti-Nb-Ta-Zr system alloys were investigated by loading-unloading tensile tests and characterizing deformed microstructures. The behavior of unloading and reloading of the stress-strain curves up to strain about 2% of Ti-20Nb-10Ta-5Zr and Ti-25Nb-10Ta-5Zr alloys is similar to that obtained in metastable β type titanium alloys where the stress induced martensite transformation occurs. This indicates that the stress and strain induced martensite transformation occurred in these alloys. Furthermore, the elastic deformation of Ti-30Nb-10Ta-5Zr alloy disobeys Hooke's law. However, the behavior of stress or strain-induced martensite transformation does not recognized in the stress-strain curve up to strain about 2% of this alloy. The main deformation mechanism up to fracture of Ti-20Nb-10Ta-5Zr alloy, Ti-25Nb-10Ta-5Zr alloy and Ti-35Nb-10Ta-5Zr alloy is identified as the deformation - induced martensite transformation of β phase to α″ phase, deformation-induced martensite transformation of β phase to α″ phase and deformation twin, and slip, respectively. The deformation mechanisms for Ti-30Nb-10Ta-5Zr alloy are not explained by slip, deformation twining and deformation-induced martensite transformation. However, the super elastic behavior observed in this alloy is expected to occur without deformation-induced martensite transformation.