The characteristics of GaN/Si(111) epitaxial layers grown by metalorganic vapor phase epitaxy(MOVPE) with AlxGa1-xN/AlN composite buffer layer(CBL) where x varies from 0.07 to 0.15 in AlxGa1-xN were investigated. The x values of AlxGa1-xN grown with various trimethyl-gallium(TMG) flow rates while feeding trimethyl-aluminum(TMA) flow constant were determined by triple crystal X-ray diffractrometry(TCXRD). The surface images of GaN/Si(I 11) epitaxy observed by SEM show some cracks, which are due to thermal mismatch between GaN and Si. According to double crystal X-ray diffractometry (DCXRD) rocking curves of GaN(0002), the crystal quality of GaN/Si(I 11) grown on Al0.11Ga0.89N/AlN CBL exhibits the best result with FWHM of 886 arcsec. PL spectrum at RT of the GaN/Si(I 11) epitaxy grown on the CBL shows a sharp band edge emission peak at 365 run with average FWHM of 45 meV, which is compared with the best result achieved so far for GaN/Si. Therefore, it can be concluded that the AlxGa1-xN/AlN CBL having suitable Al mole fraction of AlxGa1-xN layer plays an important role in improving the quality of GaN/Si(111) epitaxy by reducing the lattice and thermal mismatch between GaN and Si(I 11). (C) 2003 Elsevier B.V. All rights reserved.