Abstract: TOPEX/Poseidon (T/P) satellite altimeter data from 1992 to 1999 and in-situ measurements from 1980 to 1999 are used to analyze the upper layer thickness (ULT) in the South China Sea (SCS). The ULT is defined as the depth of the 14°C isotherm from in-situ measurements. In comparison with the ULT derived from in-situ measurements and that derived from T/P altimeter data yields a correlation coefficient of 0.89 with a slope of 1.01 and an intercept of -2 m. The basin averaged ULT derived from in-situ measurements is about 216 m with a seasonal variation of 200 m (winter) to 230 (summer). The ULT is highly correlated with the sea surface temperature (SST) in the study area except the periods of El Niño. During the normal and La Niña years, ULT is deeper (shallower) when SST is higher (lower). However, during the onset of El Niño event, a reverse result is found, that is, ULT is deeper (shallower) when SST is lower (higher).