The phase diagram of the Na2O-ZrO2 system was experimentally investigated using the classical equilibration/quenching method and differential thermal analysis (DTA) followed by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) phase analysis. For the first time, the general feature of the phase diagram between Na2ZrO3 and ZrO2 above 1773 K was revealed experimentally. The eutectic reaction between NaZrO3 and cubic-ZrO2 was found at 64.8 mol % ZrO2 at 1883 ± 3 K. Cubic-ZrO2 was stabilized by the dissolution of Na2O (maximum solubility of 7.1 mol % Na2O at 1883 ± 3 K). Based on new experimental phase diagram data in this study and thermodynamic property data of Na2ZrO3 in the literature, the Na2O-ZrO2 system was thermodynamically optimized using the CALculation of PHAse Diagram (CALPHAD) method and a consistent set of the Gibbs energy functions for all phases in the binary system was prepared.