The impact of nanoporous SiNx interlayer growth position on high-quality GaN epitaxial film was elucidated from the behavior of dislocations. The best quality GaN film was achieved when a nanoporous SiNx interlayer was grown on a rough layer, with the high-resolution X-ray diffraction rocking curve full width at half maximum for (1102 ) reflection decreasing to 223 arcs, and the total dislocation density reduced to less than 1.0×108 cm−2. GaN films were grown on sapphire substrates by metal organic chemical vapor deposition. The quality of these films was investigated with high-resolution X-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. A preference for the formation of half-loops to reduce threading dislocations was observed when an SiNx interlayer was grown on a rough layer. A growth mechanism is proposed to explain this preference.