The efficacy of wet cleaning on GaN surface to remove the metal contamination, such as Fe, Ni, Cu, and Zn, has been investigated using total-reflection X-ray fluorescence analyzer (TXRF). Metal contamination on GaN surface can be reduced by the solutions with low pH and high redox potential. The start redox potential of the metal oxidation on GaN surface is larger than that of metal itself. Because metal can receive electrons from GaN surface, and then metal cannot be oxidized to ionization by solutions. The metal contamination on GaN can be reduced by the solutions with the pH less than 4.2 and the redox potential larger than 0.8 V are required. In addition, the cleaning solution required the etching ability of β- Ga2O3, because some kind of metals include in the native oxide of GaN, which is like β-Ga2O3. This method is very useful for metal cleaning and very effective for reducing chemical consumption. analysis.