Provided is a method of manufacturing a high strength ferritic/martensitic steel. The method includes melting a ferritic/martensitic steel, hot-working the melted ferritic/martensitic steel, normalizing the hot-worked ferritic/martensitic steel at a temperature of about 1050° C. to about 1200° C., tempering the ferritic/martensitic steel at a temperature of about 600° C. or less, and leaving MX precipitates while preventing a M23C6 precipitate from being precipitated, and cold-working and thermal-treating the ferritic/martensitic steel in a multistage fashion, and precipitating M23C6 precipitates. Through the above described configuration, the high strength ferritic/martensitic steel that prevents a ductility from being deteriorated even in a high-temperature environment may be manufactured.