This study investigates depth and temporal variability of organic carbon (POC), total nitrogen (TN) and their isotopic compositions in sinking particulate organic matter collected at station KK (18¢X15¡¦N; 115¢X5¡¦E), northern South China Sea. Sinking particles collected from two separate sediment trap moorings conducted from August 8, 2004 to February 16, 2005 (KK-3) and from April 1, 2005 to October 10, 2005 (KK-4). Results show that their variations respond closely to the strong seasonality in the surface layer, but are modified considerably by subsequent remineralization and large terrigeneous input in the deep water. The highest of Al concentrations at 3500m is equivalent approximately to 58% of lithogenic material. Sinking particles of terrestrial origins has lower £_13C values of POM. Terrigeneous input should make the £_13C values increase but decrease from 600m to 3500m. Based upon data measured in this study, a C/N increase rate of 0.21 unit per 1 km of water depth. The C/N ratio of POM collected in the winter is significantly higher than those collected in the rest of the year (9.05 vs. 7.02). With this regard, cyanobacteria, which have been reported as an important N2-fixer may attribute to the insufficiency in new production sources. The £_15N values prove the occurrence of nitrogen fixation in the surface water of the South China Sea in summer.