The effects of layer thickness of TiN buffer layers on GaN growth were investigated where the TiN layers with thickness of 2, 5 and 10 nm were deposited on the sapphire substrates. The 5-nm-thick TiN layers with the finest grains produced the largest GaN grains. In addition, this TiN layers provided the smoothest surface with roughness of about 10 nm in the 2-μm-thick GaN layers. Thus, the 5 nm thickness is believed to be the best thickness for the smooth GaN growth on the sapphire/TiN substrates. The reduction of the average grain size of the TiN layer is important for the smooth GaN growth. Although the grain size was reduced with decreasing the TiN layer thickness down to 5 nm, the grain size of the 2-nm-thick TiN layer was larger than that of the 5-nm-thick TiN layer. The 2-nm-thick TiN layer provided larger surface roughness of the GaN layer than the other TiN layers and a high density of graves and holes were observed on the GaN surfaces. This suggests that the 2 nm thickness was not thick enough to relax the strain introduced due to the lattice mismatch between GaN and sapphire layers.