In this study, the isothermal sections of the Mo-Ru-Ti ternary system at 1100 and 1300 °C have been established using electron probe microanalysis and X-ray diffraction techniques. The results indicated that: (1) There were three three-phase regions at 1100 °C isothermal section and two three-phase regions at 1300 °C isothermal section; (2) the σ-Mo₅Ru₃ phase was stabilized to 1100 °C and formed a small single-phase region with the addition of Ti. (3) the (βTi, Mo) phase extended from Mo-rich side to Ti-rich side and dissolved a lot of Ru at 1100 and 1300 °C. The phase relationship of Mo-Ru-Ti system will provide the basic experimental data for the thermodynamic database of titanium alloys.