Abstract: Based on the conductivity-temperature-depth(CTD) dataga thered by R/Vs Science 1 and Shiy an 3 during the South China Sea Monsoon Experiment(SCSMEX) IOP from May through August 1998,ther mohaline properties and their variabilities along sections in the South China Sea(SCS) are discussed.The thermohaline structure in the centr al SCS is found to be mainly controlled by the SCS water masses,while that in the northern SCS, especially adjacent to the Luzon Strait,is significantly influenced by the western Pacific(WP) water in the surface and subsurface layers.A fter the monsoon onset,SST increased in the north SCS with the amplitude decreasing east-ward,while basically unchanged in the central and south SCS,hense the zonal SST gradient in the nor theastern SCS and meridional SST gradient over the basin being weakened.The SSS decreased and mixed layer developed in the northern SCS,resulting from streng thened precipitation and wind after onset of the SCS monsoon.It seems that the Kuroshio Water spread westward through the Luzon Strait with great variation.During April to May 1998,the Kuroshio subsurface water appeared in central and northern Luzon Strait and west of Luzon Island.Part of Kuroshio Water seemed to enter the SCS southw estward along the Luzon coast.During June to July,the salt core of the NPSW in the northern Luzon Strait was extremely strengthened and extended,while the one west of the Luzon Island shrinked.Although entering and depar ting the SCS with strong ervelocity,the Kuroshio Waters impact on the SCS thermal and dynamic structures may not increase.A patch of surface salt water with salinity exceeding 34.55 is found near Palawan,which seems to be originated from the western Pacific surface water through the strait between Palawan and Luzon Islands.