Diamond has been grown on 6H-SiC single crystal wafers by microwave plasma assisted chemical vapor deposition with a negative bias pretreatment. A high nucleation density of diamond on the substrate has been achieved. Cross-sectional transmission electron microscopy was employed to study the interfacial microstructure of diamond on 6H-SiC. Lattice image observations illustrate that diamond is directly formed on the 6H-SiC substrate. The possibility of local epitaxial nucleation of diamond on the 6H-SiC substrate and its crystallography are briefly discussed.