A study was conducted on biological nitrogen fixation and plow-down of green manure crops on rice yield and nitrogen dynamics in paddy field in the Taihu Lake Region, using the ¹⁵N natural abundance method. Results show that Chinese milk vetch and faba bean biologically fixed about N 32.8 and 66.8 kg km^-2, respectively, in their growth season. Plow-down of green manures as a nitrogen source basically satisfied N requirements of the subsequent rice crop. The application of N 120 kg km^-2 in addition to the plow-down could not only ensure higher rice grain yield, but also reduce the application of chemical fertilizer N by 45% to 55% compared to the common dosage in this region. The treatment of plow-down alone was significantly lower than the treatment of plow-down plus N fertilizer in concentrations of NH₄⁺-N, NO₃^--N and TN of the soil solution in the plowed layers during the whole growth season. The treatment of plow-down of faba bean was higher than the treatment of plow-down of Chinese milk vetch in TN concentration in the soil solution. Additional application of chemical nitrogen rendered the concentrations of NH₄⁺-N, NO₃^--N and TN in soil solution a rising tendency, but the effects were not much different between treatments of different application rates. Nitrogen leaching in paddy fields using green manures as the only nitrogen source for rice crop was much lower. With the application of chemical nitrogen, nitrogen leaching increased. The leaching of NH₄⁺-N、NO₃^--N and TN into deep soil water was especially higher in the treatment of 300 kg N km^-2 than in the treatment of N 0~240 kg km^-2.