Rare　earth　gangue,　which　mainly　consists　of　mixtures　of　light　rare　earths　such　as　lanthana,　ceria,　neodymium　oxide　and　praseodymium　oxide,　was　used　as　the　promoter　of　fused　iron　catalysts　for　ammonia　synthesis.　The　result　showed　that　the　activity　of　the　catalyst　promoted　with　rare　earth　gangue　was　comparable　with　those　of　commercial　iron　catalysts　with　high　amount　of　cobalt.　The　role　of　rare　earths　was　owed　to　their　advantages　for　favoring　the　deep　reduction　of　the　main　composite　in　catalyst,　i.e.,　iron　oxide.　This　finding　indicated　that　the　use　of　rare　earth　gangue　could　decrease　the　content　of　cobalt　or　even　completely　replace　cobalt,　which　was　used　to　be　regarded　as　unsubstitutable　promoters　for　high　performance　ammonia　catalyst;　therefore,　the　cost　of　fused　iron　catalysts　would　decrease　significantly.