A novel rice cultivation in aerobic soil in China has made it possible to intercrop the rice with legumes,which is one of the ways in sustainable production of rice.Pot experiments using 15 N isotope dilution method were carried out to study biological nitrogen fixation by groundnut and nitrogen transfer from groundnut to associated rice in aerobic soil at three N fertilizer application rates (15 kg hm^-2,75 kg hm^-2 and 150 kg hm^-2).Decomposition experiment of groundnut root labeled by 15 N foliar enrichment was also conducted at the same time to investigate the contributed nitrogen transfer from the decomposition of groundnut root to the associated rice.The results showed that at 15 kg hm^-2 and 75 kg hm^-2 N fertilizer application rates the dry weights of rice in intercropping condition were increased by 23.5% and 12.2% respectively compared with rice in monocropped condition.The total N accumulated in rice in intercropped was 135 and 144 mg plant^-1 at the N fertilizer application of 15 and 75 kg hm^-2 respect ively while they were 117 and 131 mg plant^-1 in monocroopped treatment respectively,showing that inter-cropping could significantly increase the dry weight and nitrogen content of rice,especially at low N application rate.The dry weight and nitrogen content of groundnut intercropped were not significantly different from that monocropped.At three N application rates,biological N fixation(BNF) rates by groundnut were 72.8%,56.5%,35.4% and 76.1%,53.3%,50.7% in mono-and intercropped condition respectively,indicating that the BNF by groundnut was significantly increased in intercropped condition and the BNF decreased with increasing N fert ilizer application rates.The N transferred rates and the amount of N trans-ferred were 12.2%,9.2%,6.2% and 16.3 mg plant^-1,13.0 mg plant^-1 and 10.4 mg plant^-1,respectively,with N fertilizer application rates of 15 kg hm^-2,75 kg hm^-2 and 150 kg hm^-2,indicated N transfer decreased with increasing N fertilizer application rates in rice/groundnut intercropping system.Using 15 N foliar labeled method,it was shown that the decomposition of groundnut root had a significant contribution to N transfer happened in groundnut associated with rice system.