Abstract: In the rice-azolla-fish cropping system as we tested after azolla had been eaten by fish, the effect of fish’s excreta for rice growth and nitrogen uptake were studied with 15N isotope labelled method. Results of experiment in the paddy fields showed that azolla was a good fodder to fish (T,nilotica). Its weight increased 41.2-48.03% during early rice season, there azolla(60 kg/ha) was put into rice-azolla-fish system as the fish’s fodder.After that, the excreta also fertilized the paddy soil and had a good effect to rice growth so the rice yield increased 9.7-20.7% than that of check (without application of N-fertilizer), but azolla (60kg/ha) was incorporated as green manure in paddy soil the rice yield only increased 4.4-14.8% as compared with check. we found azolla-N could be utilized for several times in rice-azolla-fish. First the nitrogen content of fish reached 2.41% and 15N excess was changed from 0.366% to 0.4313%. So the protein of plant was changed into animal protein effectively during early rice seaeon. Second, the rates of 15N recoveried by fish and rice were 38.24% and 29.52% respectively, there fore the total rate of 15N utilized was 67.76% in the 15N isotope labelled azolla as fish fodder and that was higher than the average rate of N utilized in azolla incorporated as green manure. The later was 48.83%.