The loss of nitrogen in the medium treated with amorphous ferric oxide was investigated with ¹⁵N-labelled ammonium sulfate in the incubation flasks under anaerobic condition. The gaseous composition of the atmosphere air in the upperspace above the same medium solution in the serum bustle was analyzed by Organic Mass Spectrometer during the experiment period. It was found that various gaseous fractions of ¹⁵N and nitrogen Loss occurred in the medium contaming ¹⁵NH₄-N due to applying amorphous ferric oxide under anaerobic condition. The gaseous Fractions emited from the medium involved ¹⁵NO₂, ¹⁵N₂O, ¹⁵N¹⁴NO, ¹⁵NO, ¹⁵N₂ and ¹⁵N¹⁴N. The rate of ¹⁵N lass was about 15%.It was primarily proved that the amorphous ferric oxide may act as an electron acceptor in the oxidation of NH₄⁺-N under anaerobic condition. It rnay be another mechanism for nicrogen loss in paddy soils.