A method of heat treating steel for use in transmission gears of a vehicle including the steps of carburizing the steel to 900-950.degree. C. quenching the carburizing steel at 830-850.degree. C., reheating the steel with ammonia gas to a temperature of 800-900.degree. C., and fusing the reheated steel at a temperature of 150-230.degree. C. for providing excellent anti-abrasion, contact fatigue strength and blending fatigue strength properties, thereby controlling the quality of steel in a furnace.