Effect of argon ion bombardment on copper oxide was investigated by in situ X-ray photo-electron spectroscopy(XPS).It turned out that CuO can be reduced under extremely weak argon ion beam condition.At the initial stage of the whole bombardment process,Cu2p XPS spectra and CuLMM Auger spectra changed drastically.The final chemical state is the coexistence state of CuO and Cu2O with a small amount of Cu by analyzing Cu2p and CuLMM Auger peaks.The important reference values were obtained for the determination of the chemical states in the depth analysis of copper oxide.