Interfacial free energies strongly influence many materials properties, especially for nanomaterials that have very large interfacial areas per unit volume. Quantitative evaluation of interfacial free energy by means of computer simulation remains difficult in these cases, especially at finite temperature. Density Functional Theory (DFT) simulation offers a robust way to compute both the energies and structures of the relevant surfaces and interfaces at the atomic level at zero Kelvin, and can be extended to finite temperatures in solids by means of the harmonic approximation (HA). Here we study the Mg/MgO interface, employing DFT calculations within the HA to obtain its key physical properties. We calculate the free energies of several key surfaces/interfaces when the temperature (T) increases from 0 K to 800 K, finding that all free energies decrease almost linearly with T. We have considered two surfaces, Mg(0001) (0.520–0.486 J/m2), and MgO(100) (0.86–0.52 J/m2), and two Mg(0001)//MgO(100) interfaces with the Mg–Mg and Mg–O stacking sequences at the interface planes (1.048–0.873 J/m2 and 0.910 to 0.743 J/m2 respectively). Using these values we determine the interfacial free energy as a function of temperature and size for MgO nanoparticles in solid Mg, an important metal matrix nanocomposite material.