A phase diagram of Ta2O5SnO2 system was modeled thermodynamically with experimental data available in the literatures. The excess Gibbs energies of the solution phases were described on the basis of the Redlich-Kister solution models. A consistent set of optimized interaction parameters was derived for describing the Gibbs energy of each phase in this system leading to a good fit between calculation and experimental data. The liquidus, solidus, and solvus were calculated and also the lattice stabilities of the components were estimated. In addition, tentative activity diagram and the metastable miscibility gaps in the liquid, a-tantala, and cassiterite phases were calculated from the present thermodynamic modeling.