By using ¹⁵N techniques, effects of urea and KNO₃ on processes and products of inorganic nitrogen transformation in Hydragric paddy soil and Gleyic paddy soil, collected from Wuxi and Yingtan, respectively, were investigated under flooding conditions at 25. The results showed that coupled nitrification and denitrification of NH₄⁺ occurred in both paddy soils. Oxidation of NH₄⁺ in the Gleyic paddy soil amended with ¹⁵N-labeled urea alone was terminated at the product of NO₂^-, but denitrification product N₂ was observed, implying a high possibility of aerobic denitrification. The decrease in NO₃^- concen-tration in both soils, followed the firstorder reaction. The reaction constant of the Hydragric paddy soil was about 10-fold that of the Gleyic paddy soil. Addition of urea raised soil pH and stimulated DNRA process in the Hydragric paddy soil, and both denitrification and DNRA processes in the Gleyric paddy soils. However, denitrification was a dominant process in NO₃^- transformation. The differences in the effects of urea and KNO₃ on the processes and products of inorganic nitrogen transformation in paddy soils would primarily result from their different effects on soil pH and secondarily from concentration effect.