We investigate the coalescent GaN nanorods grown on Si (100) and Si (111) substrates. Our results clearly show that GaN nanorods grown on both substrates have the same structural, optical and morphological properties. However, we observed a clear difference in terahertz (THz) radiation between the two sets of GaN nanorods. With high gallium molecular beam flux around 6 × 10−7 Torr, coalescent GaN nanorods grown on Si (111) substrates exhibit observable THz radiation, while the ones grown on Si (100) substrates do not. The inactive THz behavior of the GaN nanorods grown on Si (100) substrate is due to the presence of randomly-rotated GaN nanorods during coalescence. The dissimilarity in THz radiation behavior between the two GaN nanorods, i.e. interfering incident optical pulse thus exhibiting inactive THz radiation from GaN nanorods grown on (100) substrate indicate that the nanorods are attractive for further THz applications not limited to III-N materials system but also other materials systems.