GaN thin layers (200 Angstrom) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates, Transmission electron microscopy reveals that two different epitaxial relationships may occur, The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and [<1(1)over bar 00>](GaN)parallel to[<11(2)over bar 0>](Al2O3) is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: [<11(2)over bar 0>](GaN)parallel to[<1(1)over bar 00>](Al2O3) and [<1(1)over bar 03>](GaN)parallel to[<11(2)over bar 0>](Al2O3). This corresponds to a tilt of about 19 degrees of the c axis with respect to the substrate surface. (C) 1997 American Institute of Physics.