Catalytic NH3 decomposition with limonite rich in alpha-FeOOH has been studied as a hot gas cleanup method to remove a low concentration of NH3 from fuel gas produced in coal gasification. Fine particles of metallic Fe formed from alpha-FeOOH achieve the almost complete decomposition of 2000 ppm NH3 to N-2 at 500 degrees C under a high space velocity of 45,000 h(-1) and show the stable catalytic performance for 50 h. The predominant catalytic mechanism involving the formation and subsequent decomposition of Fe nitrides may be proposed on the basis of the XRD and TPD results.