The microstructures and the residual-stress of thin-cubic GaN epilayers grown on Si(0 0 1) substrate coated with a thin flat SiC buffer layer were studied. The thin SiC layer is an effective buffer layer for GaN growth. Near the GaN/SiC interface there are many stacking faults, micro-twins, and micro-hexagonal phase induced by large compressive stress. All these planar defects do not introduce energy levels in the band gap that are responsible for the yellow-band luminescence. The compressive stress caused by the lattice mismatch between GaN and SiC is essentially relaxed within several tens of nanometer from the interface. The critical film thickness for full strain relief is about 100 nm.