We have grown GaN epilayers, with a thickness of 1.3-1.5 mu m, on 100 mm diameter Si(lll) substrates. The GaN films were grown by using buffer layers consisting of 3C-SiC and 2H-AlN. The buffer layers had Aat surfaces without showing any grain microstructure. The GaN films were also flat and smooth without noticeable microcracks but had slip lines. X-ray diffraction and electron diffraction analyses showed that each of the buffer layers and the GaN film were single-crystals. The room temperature PL showed that the FWHM was as low as 35.5 meV, the narrowest for all reported GaN/Si films grown by blanket MOCVD. Voids on the top of a Si substrate were generated during the conversion of the Si surface to 3C-SiC. The voids may facilitate the Si top surface to act as a compliant layer to alleviate the film stresses resulting from mismatches in lattice parameters and thermal expansion coefficients between the films and substrate. This may be the mechanism that prevented films from cracking.