The interface in Si-on-sapphire studied by transmission electron microscopy (TEM) and X-ray diffraction techniques showed chat the complicated interfacial structure is associated with arrays of microtwins and stacking faults as well as interfacial dislocations. The networks of dislocations were found to have non-rational line directions. The elastic strain in the heterostructure is accommodated by these defects and crystallite misorientations at the interface.