Abstract: By the method of Silva and Bremner,the fixed ammonium content of chief paddy soil types in Hunan Province and its influencing factors were studied. The results showed that the content of fixed ammonium in plough layer ranged from 141 mg·kg^-1 to 353 mg·kg^-1,averaged 272±67 mg·kg^-1,and accounted for 112% of total soil N. The content of fixed ammonium was in order of alluvial sandy soil > purple clayey soil > newly alluvial sandy soil > yellow clayey soil > reddish yellow clayey soil. There were four distribution models of fixed ammonium in the soil profiles: 1) fixed ammonium content increased with the increase of depth in the profiles; 2) fixed ammonium content decreased with the increase of depth; 3) there was no distinct change; and 4) abrupt increase or decrease was found in the profiles. The percentage of fixed ammonium to total Nin soils always increased with the increase of depth. The fixation of NH₄⁺ by soil was the strongest under 30℃. Long-term submergence benefited to the fixation of NH₄⁺ in newly alluvial sandy soil,purple clayey soil,and alluvial sandy soil,while the alternation of drying and wetting contributed to the fixation of NH₄⁺ in yellow clayey soil best. The content of fixed ammonium in tested soil was correlated with the content of clay <0.01 mm at the significant level,and not correlated with the content of clay <0.001 mm,total N,organic N,and matter.