The application of 234 Th- 238 U disequilibria to study the particle dynamics in the euphotic zone of the South China Sea during the 2002 spring cruise had been made.The contents of the particulate organic carbon (POC),234 Th (dissolved and particulate) and 238 U at three stations were determined and profiles of 234 Th/ 238 U) A.R.in water column were discussed.Profiles of dissolved and particulate 234 Th and a steady state box model were used to quantify dissolved 234 Th scavenging fluxes,particulate 234 Th removal fluxes and their resident times.The POC export fluxes out of the euphotic zone (export production) ranged from 20.59~22.74mmol/(m 2 ·d) and 19.22~45.28mmol/(m 2 ·d).The agreement of the results derived from two different approaches showed that 234 Th was an excellent tracer for carbon cycle.