This study demonstrates an easy and fast method of controlling the average diameter and decreasing the standard deviation (SD) of Au nanoparticles (NPs) and GaN nanowires (NWs). Plasma, immersion, and heat-treatments were conducted before the growth of GaN NWs. After the plasma treatment of the Si(111) substrate, immersion treatment was used to change the diameter of Au NPs adsorbed on the substrate surface. Results indicated that the Au NP diameter can be controlled using the immersion time. The Au NPs had an SD of 8 nm, which is much lower than that of a high temperature annealed thin film (20 nm). The experimental results indicated that the GaN NW diameter depended on the Au NP diameter. The GaN NW diameter was in the range of 27-44 nm. The GaN NWs had an SD of 10 nm, whereas GaN NWs grown from a high temperature annealed thin film had an SD of 35 nm. X-ray photoelectron spectroscopy and NMR analysis spectra showed the Si-C bond and carbon-carbon double bond on Au NPs, respectively. The X-ray diffraction pattern confirmed the hexagonal structure of GaN NWs. The blueshift of photoluminescence emission was verified when the NW diameter decreased from 89 to 31 nm.