PURPOSE: To economically and practically remove oxygen from a hydrocarbon fuel and to improve thermal stability of the hydrocarbon fuel by a constitution wherein a strong reducing agent is continuously introduced into the hydrocarbon fuel and the fuel and the reducing agent are brought into enough and effective contact with each other.

CONSTITUTION: The following method is used for improving thermal stability of a hydrocarbon fuel used as a cooling medium in a means of transportation mooing at an extremely supersonic speed. Namely, a process for introducing continuously a flow of a liq. consisting of a strong reducing agent with an enough amt. for reacting with oxygen dissolved in the above described fuel into this fuel, and a process for mixing thoroughly the reducing agent and the fuel and dispersing mutually uniformly them so as to bring the reducing agent and the fuel into close contact with them together, are provided to decrease the level of the dissolved oxygen existing in the above described hydrocarbon fuel to a level of approximately 5 ppm. As the above described reducing agent, hydrazine or its deriv. is pref. and introduction of the reducing agent is pref. performed on the upstream side of a fuel pump.