Ocean Drilling Program Site 807 is located on the Ontong-Java Plateau, western equatorial Pacific. Quantitative analysis of planktonic Foraminifera, combined with oxygen and carbon isotope data, reveals glacial-interglacial variations in sea surface temperature and upper water vertical structure during the late Quaternary in this region. Our results indicate that, since 530 ka BP, sea surface temperature (SST) and the depth of the thermocline (DOT) have changed significantly in the West Pacific Warm Pool (WPWP). The average glacial-interglacial annual SST difference was up to 4.2 degrees C, and DOT fluctuations could exceed 100 m, further suggesting the instability of the WPWP. Spectral analyses of SST and DOT reveal two dominating cyclicities: typical 100 ka glacial-interglacial periodicity and some semi-processions found only in the tropics, indicating that late Quaternary paleo-oceanographic changes in the study area have been influenced not only by high latitude forcing but also by tropic-driving factors.