Thin films of Au were deposited onto MgO substrates cleaved in ultrahigh vacuum. The films, which were examined by transmission electron microscopy and electron diffraction, did not grow with a (111) orientation observed in a Au/NaCl system but with a (100) orientation which had been observed in a Pd/MgO system. In order to obtain some insight into the differences in the epitaxial orientations, we studied interfacial chemical bonding states in Au/MgO(100), Pd/MgO(100), and Au/NaCl(100) systems using total pair-potential calculations and discrete variational Xα calculations. The results show that interfacial interaction in both the Au/MgO and the Pd/MgO system is stronger than that in the Au/NaCl system. The relation between the strength of the interfacial chemical bond and the epitaxial orientation in the above systems is discussed.