Mechanism of a recently reported new etching method of transition metal using ethanol gas and oxygen and argon neutral beams at low temperature was investigated by first-principles calculation based on density functional theory. Tantalum etching was studied. It was found that adsorption of ethanol on both of tantalum oxide and metallic tantalum. Then 'hydrogen movement' reaction can occur by argon neutral beam bombardment, and it weaken chemical bonds at the oxide surface. The etching process is like following: (1) oxidation of metal surface, (2) adsorption of ethanol, and (3) hydrogen movement. Especially, the 'hydrogen movement' is the reason why oxidation is needed. Such understanding should be useful to design new etching processes using transition metal complex.