Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium–aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (a) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium a is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt–vapor systems based on first-principles molecular dynamics and the hightemperature approximation of the Bigeleisen–Mayer equation. We found that, at 2500 K, d25Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141 ± 0.004 and 0.143 ± 0.003% more positive than in their respective vapors. The corresponding d26Mg values were 0.270 ± 0.008 and 0.274 ± 0.006% more positive than in vapors, respectively. The general a T equations describing the equilibrium Mg a in MgSiO3 and Mg2SiO4 melt–vapor systems were: aMg lð ÞMg gð Þ ¼ 1 þ 5:264105 T2 1 m 1 m0 and aMg lð ÞMg gð Þ ¼ 1 þ 5:340105 T2 1 m 1 m0 , respectively, where m is the mass of light isotope 24Mg and m0 is the mass of the heavier isotope, 25Mg or 26Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.