Thin film growth of Zn x Mn1-x S on a Si (100) substrate by sputtering was investigated for nonpolar AlN film growth on Si (100) substrate. The Zn x Mn1-x S buffer layer reduces the large differences in thermal expansion coefficient and lattice constants between AlN and Si. Although the solubility of ZnS in MnS is less than 5% at 800 °C in bulk form, the insertion of a room-temperature MnS layer between Zn x Mn1-x S and Si enabled (100)-oriented cubic-Zn x Mn1-x S film growth even at x = 9.5%, which is a metastable phase and a phase separation region in bulk form. On the (100)-oriented cubic Zn x Mn1-x S film, nonpolar AlN growth was achieved by sputtering. Furthermore, X-ray photoelectron spectroscopy measurements revealed that the Zn x Mn1-x S film improved the stability of the AlN/Zn x Mn1-x S interface. Zn x Mn1-x S has the potential to enable nonpolar AlN growth on large-diameter Si (100) substrates.