Abstract: This paper deals with a new process of rapidly cooling blackening. Its treatments are as follows:the parts are heated in air atmosphere in a furnace at the temperature 500-600℃ for a given time and followed by quenching in cooling liquid to room temperature. A black oxide coating on the parts can be obtained when they are moved out from liquid. The composition of liquid is 25-28%mineral oil, 1-2% NaNO₂, 0.2-0.3% Na₂CO₃ and 70%water. It shows in practice that this technology not only improves corrosion resisitence,but also increases mechanical properties,surface hardness and strength.