Abstract: The monthly input-output flux of nitrogen as well as its variation characteristics under acid deposition at Caijiatang small forested catchment in Shaoshan City,Hunan Province was examined,based on the monitoring data of bulk precipitation,throughfall and runoff during 2001.Results showed that the N-input flux in 2001 was 53.81 kg/hm²,in which the flux of NH₄⁺ accounted for 64%.The N-output flux in 2001 was 3.57 kg/hm²,in which the flux of NO₃^-accounted for 64%,which indicated a net retention of 50.24 kg/hm² in CJT catchment.The maximum monthly flux occurs in May,June and July,with the total N-output in these three months accounting for 59% of the total N-output in the whole year.In general,the output of N does not increase along with the increasing input of N,but is mostly influenced by the rainfall quantity.