We measured the synchrotron X-ray diffraction during the growth of GaN films on silicon carbide (SiC) substrates (a). With increasing GaN-film thickness, the diffraction peak of GaN becomes larger and its peak position is gradually shifted as indicated in (b). The film-thickness dependence of the peak position is shown in (c). It is clear that a lattice spacing along the L-direction shifted more drastically than that along the H-direction, indicating a difference from conventional elastic theory. “r.l.u.” indicates reciprocal lattice unit and the large (small) value corresponds to the small (large) lattice spacing of GaN.