Disclosed is a method for removing oxygen from aluminum nitride by carbon. At first, an oven is provided. An aluminum nitride substrate is located in the oven. Nitrogen is introduced into the oven to form an atmosphere of nitrogen. The temperature is increased to the transformation point of the aluminum nitride substrate in the oven. Then, the heating is stopped and quenching is conducted in the oven. Carbon is introduced into the oven in the quenching. Thus, oxygen included in the aluminum nitride substrate reacts with the carbon to produce carbon monoxide or carbon dioxide. The carbon monoxide or carbon is released from the oven as well as the nitrogen. Thus, the aluminum nitride substrate is purified.