Abstract: Ammonia synthesis at supercritical conditions was first studied over iron and active carbon-supported ruthenium catalysts in a fixed-bed reactor.The influences of 15 kinds of different supercritical media,such as alkanes of C7—C13, 1,2,3,4-tetrahydronaphthalene,cis-decalin,o-xylene,ethylbenzene,quinolin,n-hexane and aniline etc.and reaction conditions (catalyst, temperature, space velocity, particial pressure of media) on ammonia at supercritical condition were investigated.Supercritical medium was decomposed under reaction conditions over Fe and Ru/AC catalysts.The decomposition products deactivated the catalysts.Alkane decomposed the least,and the rate of deactivation was the slowest.Therefore alklane was a relatively good medium.The decomposion of supercritical medium was the key for the deactivation of catalysts.Another important reason for the decrease of ammonia concentration was that the effective pressure of syngas decreased because of the presence of supercritical media.The active temperature of catalyst was the decisive factor in supercritical ammonia synthesis.Supercritical catalytic reaction was viable only at a lower temperature.Ammonia synthesis at supercritical conditions is possible if a catalyst with active temperature lower than 573 K could be developed and the decomposition of supercritical media could be prevented.