Microstructures, mechanical properties, and crystal structures of Ti-(50-52)mol%Al alloys containing Sn up to 5mol% as a third element have been studied by using an optical microscope, a transmission electron microscope, a micro-Vickers hardness tester, and an X-ray diffractometer. Results obtained are as follows: The amount of Ti₃Al (α₂) phase existing in as-cast alloys was decreased by addition of Sn and it disappeared by anannealing for 604.8ks at 1273K. But the Ti₃Sn phase formed in the cast alloys containings Sn did not disappear by the annealing. The Ti₆Sn₅ phase was formed in the Ti-50mol%Al-5mol%Sn alloy by the annealing. Both of the lattice parameter c and the axial ratio c/a of the Ll_O-type Al-rich intermetallic compound TiAl (γ phase) increased with increasing Sn content. Tin atoms in the γ phase occupy predominantly in the Ti site. The amount of mutual exchanges of Ti and Al atoms in lattice sites increased with increasing Sn content in the γ phase.