High quality epitaxial MgO thin films have been grown on Si (001) wafers by molecular beam epitaxy using SrTiO3 (STO) as a buffer layer. The STO buffer layer reduces both the large lattice mismatch of 23% and the large thermal mismatch of 520% between MgO and Si. X-ray diffraction (XRD) measurements indicate that the MgO film grown on the STO buffered Si is epitaxial with MgO (002)||Si (004) and MgO ||Si . The full width at half maximum (FWHM) of MgO (002) rocking curve width Δω is 0.30° (out-of-plane), and the FWHM of MgO (202) Φ angle scan width ΔΦ is 0.34° (in-plane) for a 155 nm thick film. Strain relaxation and growth mechanisms of the MgO film on Si were studied by in situ reflection high-energy electron diffraction (RHEED) analysis in combination with XRD and atomic force microscopy. The results indicate that the MgO first forms a pseudomorphic wetting layer and subsequently undergoes a Stranski-Krastanov transition to form three-dimensional coherent islands to relieve misfit strain. A decrease in the width of the RHEED spots with increasing MgO thickness is observed that is attributed to reduction of coherency strain. A smooth surface redevelops once MgO growth continues, which is attributed to island coalescence.