Abstract We report on the influence of different surface orientations of Si substrates on the properties of GaN-based layer structures grown by metalorganic vapor-phase epitaxy. By using a high-temperature AlN seed layer, a monocrystalline and c-axis-oriented GaN-layer can be obtained on all substrates with Si(1 1 0), Si(1 1 1), Si(1 1 5), Si(1 1 7), and Si(1 1 9) surface planes. In particular, the samples on Si(1 1 0) substrates exhibit a high quality of the GaN layer, which is comparable or even better than that of identically grown test structures on standard Si(1 1 1) substrates. This result can be explained by a more suited epitaxial relation between the c-plane of the high-temperature AlN seed layer and the Si(1 1 0) surface. The crystallographic structure of approximately 500-nm-thick GaN layers on Si(1 1 0) and Si(1 1 1) are analyzed by X-ray diffraction measurements, their surface morphologies by atomic force microscopy, and the optical properties are investigated by photoluminescence measurements. The improved crystallographic quality of GaN on Si(1 1 0) comes along with a more efficient effect of strain compensating interlayers in comparison to GaN layers on Si(1 1 1).