All available thermodynamic properties and phase diagram data of the Sn-O system at 1 atm total pressure were critically evaluated and optimized to obtain a set of thermodynamic functions of all stable phases. The discrepancies among available experimental phase diagram data were resolved. In addition, new experiments were attempted to determine the melting temperature of SnO2 using sealed Pt capsules. It was confirmed that the melting temperature of SnO2 is higher than 1700 °C, which was taken into account in the present optimization. The Gibbs energies of the liquid solution of SnO-SnO2 and all solid compounds were obtained. A new phase diagram at 1 atm total pressure was constructed from the optimized Gibbs energy functions of all phases.