Microstructures and tensile properties were investigated in metastable beta-type Ti-10Mo, Ti-15Mo, and Ti-20Mo alloys after solution treatment. In addition to beta phase, different martensitic structures with various sizes and morphologies were found in these alloys. In the Ti-10Mo alloy, acicular alpha aEuro(3) plates were uniformly distributed in the beta matrix. Transmission electron microscopy (TEM) revealed {111} alpha aEuro(3)-type twins had formed in alpha aEuro(3) plates, and the orientation relationship between the beta and alpha aEuro(3) is close to (110) beta//(001) alpha aEuro(3), () beta//(110) alpha aEuro(3), and [] beta//[] alpha aEuro(3). In the Ti-15Mo alloy, nanoparticles of athermal omega phase containing {112} beta-type twins were observed in the beta matrix. Smaller omega particles were found in the Ti-20Mo alloy than in the Ti-15Mo alloy. When tensile tested, the Ti-10Mo and Ti-15Mo alloys exhibited large plastic strains of 24 and 29 %, respectively, with ultimate tensile stresses of 756 and 739 MPa. The Ti-20Mo alloy displayed a higher ultimate tensile stress of 792 MPa but much smaller plastic strain (2 %). Although beta phase dominates in all the metastable beta-type Ti-Mo alloys, the nanostructured martensites also play an important role in the mechanical properties of the alloys.