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Wurtzite GaN epilayers directly grown on 4H-SiC (0001) misoriented by 0, 3.5, 5, 8, and 21° with plasma-assisted molecule beam epitaxy have been studied using variable-temperature photoluminescence. A strong emission peak locating at energy position -70 meV lower than the near band-edge emission peak at 3.47 eV is found in the emission spectra of the GaN films on 4H-SiC misoriented by 8 and 21°. It is clear that one type of structural defect leads to the peak. Stacking mismatch boundaries are supposed to be the candidate causing the optical transition. Combined with the low-temperature photoluminescence excitation spectra of the films, the location of the electronic level induced by the structural defect is determined to be about 104 meV above the valence-band maximum of GaN.