The Holocene variability in sea surface and thermocline water temperatures (SST and TWT) in the Indo-Pacific Warm Pool (IPWP) has been reconstructed by shell Mg/Ca of two planktonic foraminifera species, the mixed-layer dweller Globigerinoides ruber and the thermocline dweller Pulleniatina obliquiloculata, from sediments of Core MD98-2188 (14.82°N, 123.49°E, 730 m water depth) recovered in the western tropical Philippine Sea. Afterward the Younger Dryas interval (YD), SST warmed gradually till ~10 ka and remained approximately constant afterwards, but TWT rose more rapidly to a peak between ~12 and ~10 ka and then declined by ~1.5°C through the Holocene. The trend of TWT closely followed the boreal summer insolation and could be correlated to tropical climate changes represented by southward movement of the Inter-tropical Convergence Zone (ITCZ) and related changes in East Asian monsoons.