The phase transformation behavior and microstructure of Nb-Ru alloys have been studied by DSC, X-ray diffraction, optical microscopy, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Two-step phase transformation of CsCl (β)→face-centered tetragonal (β)→monoclinic (β") occurs during cooling from high temperature to room temperature. The lattice parameters of martensites of Nb-Ru alloys were found to increase with the increase of Nb content. The martensite variants exhibit triangular self-accommodating morphology, with alternating regular bands inside. The twinning relationship between the substructural bands was found to be (101) type I mode, and this kind of twinning interface was straight, well-defined and coherent.