To aim the problem of nitrogen fertilizer overused to cause lower efficiency of fertilizer-N ut ilization and environment pollution, the field and lysimeter experiments under different amount of fertilizer-N applied were conducted in the Taihu lake area to seek the optimum nitrogen rate for a high productive ricewheat system and reducing nitrate leaching.The preliminary results showed fertilizer-N rate used increased with yield of rice & wheat.Optimum rates of fertilizer-N ranged from 225 to 270 and 180 to 225 N kg hm^-2 for rice and wheat in the two main paddy soils, respectively.With both high percentage of earbearing tiller and setting percentage, yields of rice & wheat were higher at optimum rate of fertilizer-N used than at other treatments.Nitrogen was leached away from field mainly in the form of NO₃^--N, not NH₄⁺-N, and it occurred in the season of growing wheat and the early stage of rice pouding and transplanting.Nitrate-N concentrations in the leachate increased gradually with the increasing of fertilizer-N applications.Which in the lysimeter leachate ranged from 5.4 to 21.3 mg L^-1 from the field receiving 225 kg hm^-2, with 60% leachate being over the standard of 10 mg L^-1 NO₃^--N standard and in groundwater ranged from 1.9 to 11.0 mg L^-1 from the fields receiving 270 to 315 N kg hm^-2, with 20% samples being nearly and 10% over the standard, respectively.Although NO₃^--N levels in groundwater from fertilized fields are usually well below the standard, cumulating of high NO₃^--N leachate will be a potential risk of water contamination in Taihu Lake area.