Nitrogen leaching loss from double cropping rice fields was observed using a lysimeter system during 1991-1994. The main results are summarized as follows:1.The basic form of nitrogen leached from rice fields-was nitrate nitrogen(NO₃^--N) rather than ammonium nitrogen (NH₄⁺-N),so the N fertilizer applied in rice fields could result in the nitrate pollution to underground water due to N leaching.2. N application showed an obvious effect on NH₄-N concentrations in the percolateing soil depth of at 10-16cm, and a rather high NH₄⁺-N concentration occurred in urea plot compared with the NH₄HC0₃ plot, suggesting the possibility of more N-leaching in NH₄HC0₃ plot than in urea plot.3. The NO₃^--N level in the percolating at water soil depth of 90-96cm was consistently higher in the NH₄HCO₃ plot than in the urea plot, which may be ascribed to the obvious dif ference of transformation dynamics between NH₄HCO₃ and urea. 4. Under the conditions of an application rate of 300 kg urea-Nha^-1 and a percolation of 540 mm, a NO₃^--N leaching of about 23 kg ha^-1 and a NO₃^--N+NH₄⁺-N leaching load of nearly 27 kg ha^-1 were estimated for double-rice cropping paddy. 5. Based on the observed peak levels of 9 to 11 mg NO₃^--N L^-1 during a certain period of time and the limit of 10 mg NO₃^--N L^-1 in drinking water derived from underground water, we recommended a application limit of 150 kg Nha^-1 per rice-cropping season.