Abstract: The oxygen isotope value (δ¹⁸O) of foraminiferal shell depends upon the δ¹⁸O and temperature of sea water. The seawater temperature, therefore, can be extracted from foraminiferal δ¹⁸O if the δ¹⁸O composition of sea water is known. In this paper, the planktonic foraminifera Globigerinoides ruber picked out from the nine sediment cores in the northern South China Sea were analyzed for δ¹⁸O. Using the published summer and winter sea water δ¹⁸O values at each site, we calculated the sea water temperature (T_ruber), and then comparison was made for T_ruber and the 0~30m water temperature download from the World Ocean Atlas (WOA) database. Our data shows that there is a large difference between T_WOA and T_ruber when summer water δ¹⁸O was used in calculation, however, the difference was in a reasonable range when winter water δ¹⁸O was used. It suggests that δ¹⁸O_ruber reflects better the temperature and δ¹⁸O composition of winter seawater, and further implies that the productivity and deposition flux of G.ruber may reach the maximum when the East Asian Winter Monsoon prevails.