The effect of de bias voltage on the microstructure of bias-enhanced diamond nucleation layer is systematically investigated by using high resolution cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy. It is revealed that the formation of epitaxial beta -SiC phase on a silicon substrate at -200 V is responsible for maintaining the heteroepitaxial relation between the diamond film and the silicon substrate. On the other hand, amorphous nucleation layers are detected on bias-treated silicon substrate at higher dc voltages, which prevent the deposited diamond films from having the orientation of a silicon substrate. Based on the experimental results, the variation of microstructure in the bias-enhanced diamond nucleation layer with respect to the amount of dc voltage is intensively discussed. In addition, a mechanism for the formation and growth of epitaxial beta -SiC phase during bias treatment is proposed.