Ocean Drilling Program (ODP) Site 807 A was recovered from the Ontong-Java plateau, western equatorial Pacific. Quantitative analysis of planktonic foraminifera, combined with oxygen and carbon isotope data, reveals the glacial-interglacial variations of sea-surface temperature and the upper water vertical structure in this region during the late Quaternary. Our results indicate that since 530 ka sea-surface temperature (SST) and the depth of thermocline (DOT) have changed significantly in the western Pacific warm pool (WPWP). The average glacial-interglacial annual SST difference was up to 4.2 degrees C, and the DOT fluctuations could exceed more than 100 m, further suggesting the instability of the WPWP. The spectral analyses of SST and DOT reveal two dominating cyclicities--the typical 100 ka cycle and the semi-precessional cycle, which is significant in the tropical spectrum, indicating that late Quaternary paleoceanographic changes in the study area were influenced not only by a high latitude forcing but also by tropic-driving factors.