The properties of mullite-glass and mullite-mullite interfaces have been investigated at 1800 K by molecular dynamics (MD) simulation and high-resolution TEM. The simulation showed that mullite-glass interfaces typically have much lower interfacial energies than mullite-mullite interfaces, which results from the structural flexibility of the glass and associated accommodation of interfacial mismatch. The (110)-glass interface has the lowest energy of all interfaces studied, which is consistent with the observed dominance of this interface in experimental mullite-glass samples examined by TEM. The simulation shows that the interfacial energies of the (100)-glass and (010)-glass interfaces are higher than that those of the (001)-glass interface, so [100] and [010] would be expected to be the dominant growth directions. However, the growth of mullite in glass occurs predominantly in the [001] direction. This apparent discrepancy can be explained by the fact that growth in the [100] and [010] directions is limited by the slow growth of (110) plane (i.e., [110] direction), which facilitates [001] growth, which is confirmed by the TEM data.