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Calcareous nannofossil records in the sediments from ODP Hole 806B located in Ontong Java Plateau comprising Reticulofenestra size variations, Discoaster relative abundance and nannofossil accumulation rates have been analyzed to reconstruct surface water dynamics in the western equatorial Pacific throughout the Miocene. From early to middle Miocene (18 – 14.4 Ma), the surface water was relatively warm and depleted in nutrient associated with warm condition during the Middle Miocene climatic optimum. From middle to late Miocene (14.4 – 8.8 Ma), the thermocline deepened as the western Pacific warm pool established. During the late Miocene (8.8 – 5.6 Ma), the surface water suddenly shifted into cooler and nutrient rich condition associated with enhanced equatorial upwelling. The timing of this event is coincided with the intensification of Asian monsoon.