Uniformly distributed ZnO nanowall network structures were grown at 550 degrees C by organometallic chemical vapor deposition technique on the GaN/sapphire substrate without using any catalysts. In this research, we discussed the nanostructures and optical properties of ZnO samples grown under the same conditions but on different underlying materials (GaN/sapphire and sapphire). By adjusting the growth parameters, ZnO nanowall networks with a honeycomb-like pattern without using any metal catalysts were successfully fabricated on the GaN/sapphire and sapphire substrates. Since the lattice mismatch between ZnO and GaN is only about 1.8% while the lattice mismatch between ZnO and sapphire is about 18.4%. Lattice mismatch may not be the decisive factor in the formation process of ZnO nanowall networks. The ZnO grown on GaN epilayer had smaller full width at half maximum value than that of ZnO grown under the same growth condition on the sapphire substrate, indicating a higher crystal quality in the sample of ZnO on GaN. The room temperature PL measurement of both ZnO nanostructures grown on GaN and sapphire show strong ultraviolet peak intensity and high intensity ratio of the near band emission to the deep-level emission in a PL spectrum.