Polished, highly oriented diamond (HOD) with an RMS roughness of less than 3 nm was bonded to single-side polished silicon wafers in ultra-high vacuum (UHV) at 32 MPa of applied uniaxial pressure. Successful fusion of HOD to silicon was achieved at temperatures above 850 o C. Fusion resulted in the formation of an abrupt interface between the wafers in the areas away from diamond grain boundaries. Voids, partially filled with amorphous material, were observed at the fused interface near the diamond grain boundaries. Preferential diamond polishing, potential out-diffusion of hydrogen from diamond and oxygen from silicon are believed to have contributed to the observed non-uniformity of the bonded interface.