Transmission electron microscopy (TEM), transmission electron diffraction (TED), atomic force microscopy (AFM), and scanning electron microscopy have been used to investigate the initial nucleation process and growth behavior of diamond films by microwave plasma chemical vapor deposition. TED examination revealed epitaxial relations between the beta-SiC and the Si, and the diamond and the beta-SiC, which depended on the bias-enhanced nucleation (BEN) time and methane (CH4) concentration. The highly oriented (001) diamond films were obtained after 25 min BEN for 4% CH4 and 20 min BEN for 8% CH4. TEM revealed the beta-SiC crystallites 2-25 nm across and the diamond crystallites 34-40 nm in size, which depended on the CH, concentration and the BEN time. As the BEN time increased, the density of the beta-SiC cyrstallites increased from similar to 2.7 x 10(11) to similar to 3.4 x 10(12) cm(2), while that of the diamond crystallites varied from similar to 2.0 x 10(9) to similar to 4.0 x 1(01)0 cm(-2). Discrepancy between the densities obtained using TEM and AFM is discussed. It is shown that the heteroepitaxially oriented diamond crystallites are critically important for the growth of the highly (001)-oriented diamond films, although the heteroepitaxially oriented beta-SiC crystallites could serve as nucleation sites for the growth of the diamond films.