High quality GaN epitaxial layers were grown on 6H-SiC substrates by using low-pressure metalorganic chemical vapor deposition method. Samples employing a three-period GaN/Al0.08Ga0.92N (100 Å/100 Å) as a buffer layer produce a good quality GaN epitaxial layer, with mobility and carrier concentration of 612 cm2/V·s and 1.3×1017 cm-3 (at 300 K), respectively. The enhanced electron mobility in the Al0.08Ga0.92N/GaN heterostructures is also observed. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility at 4.2 K for the Al0.08Ga0.92N/GaN heterostructure are 5.8×1012 cm-2 and 5300 cm2/V·s, respectively. Strong SdH (Shubnikov-de Haas) oscillations were observed to confirm the two-dimensional electron gas (2DEG) phenomenon at the AlGaN/GaN top heterointerface. In addition, an extra SdH oscillation also resulted from the high-quality 2DEG channel of the GaN/AlGaN bottom heterointerface.