The N - labeled technique was used to study the stock, transformation, fate and utilization efficiency of N in the farming-pig husbandry-biogas ecosystem in rice areas. It was shown that the crude protein digestibility of the ensilaged milk vetch by pig was 53.76%, the recovery rates of ensilaged milk vetch N from the pig feces and urine were 39.36% and 24.71%, respectively. The recovery rate of biogas fermentation N of pig feces and urine was 97.9%. The quantity of alkali-hydrolysable N was 2.6 times as great as before the fermentation. The mixed application of milk vetch, biogas-tank sludge and chemical fertilizer could promote N partitioning to rice grain, therefore it was advantageous to yield improvement. The residual quantity in soil of the fertilizer N was equivalent to 2.0 - 2.5 times that of only chemical fertilizer and yet gaseous loss N of the latter was 2.6-8.2 times that of the former. The comprehensive economic effect of the whole ecosystem increased by times, compared with the only milk vetch-early rice-late rice pattern, and ecological effect and social effect were also very prominent.