Isothermal section of Ti-Nb-Sn at 1173 K was experimentally studied by back-scattered electron, electron probe microanalysis and X-ray diffraction analysis. Solid solution phase β(Ti, Nb), liquid Sn and eight intermetallic compounds Ti₃Sn, Ti₂Sn, Ti₅Sn₃, Ti₆Sn₅, Nb₆Sn₅, Nb₃Sn, Ti₃Nb₂Sn₂ and Ti₃NbSn coexisted. Four ternary phase regions Ti₃Sn + Ti₃NbSn + β(Ti, Nb), Ti₃NbSn + Ti₃Nb₃Sn₂ + Ti₃Sn, Ti₂Sn + Ti₃Sn + Ti₃Nb₃Sn₂ and Ti₆Sn₅ + Ti₃Nb₃Sn₂ + Nb₃Sn were experimented. In addition, the proper composition range of the single phase was suggested. All the detected Ti-Sn and Nb-Sn compounds have a remarkable solubility along the isoconcentration of Sn. β(Ti, Nb) has a relatively large solution while liquid Sn has a little in the isothermal section.