GaN thin films were grown by metal organic chemical vapor deposition on Al 2O 3(0001) substrates, the epitaxial relationships between GaN and Al 2O 3(0001) were examined by a X ray(synchrotron radiation source) diffraction method. Results revealed that GaN grown on the well nitridated substrates exhibited the popular crystallographic orientation with direction perpendicular to the Al 2O 3(0001), while those grown on the bare or insufficiently nitridated substrates proved to have three other orientations which rotated by certain angles around its <1120>in respect to the popular one. The orientation tiltings were suggested to be one of the inportant mechanisms by which GaN films could match better with Al2O3(0001) substrates and relax the interfacial strain effectively.