Metastable β-type titanium alloys have many features such as high specific strength, good ability of plastic deformation, excellent corrosion resistance, biocompatibility, and low Young's modulus, which are useful in aerospace, biomedical and chemical industries. However, plastic deformation behaviors of metastable β-type titanium alloys under multi-axial compressive conditions have not been clarified. In the present study, four types of Ti-Nb alloys, Ti-20mass%Nb, Ti-30mass%Nb, Ti-35mass%Nb, and Ti-45mass%Nb were prepared. The uniaxial and biaxial compression tests with proportional strain paths were then performed on Ti-Nb alloys by using a biaxial compression testing machine. The results of uniaxial compression tests revealed that stress induced martensite transformation, deformation twin, coarse slip occurred on Ti-20 and 30Nb, Ti-35Nb, and Ti-45Nb, respectively. It was found by biaxial compression tests that the plastic deformation behavior including anisotropic hardening also changes with Nb content.