During Ocean Drilling Program (ODP) Leg 186, we drilled six holes at two sites, Sites 1150 (39°11´N, 143°20´E) and 1151 (38°45´N, 143°20´E) on the deep-sea terrace of the landward side of the Japan Trench (Fig. F1) (Sacks, Suyehiro, Acton, et al., 2000). These sites are located under the influence of the Oyashio Extension, the northwest boundary current of the Subpolar Gyre in the Pacific Ocean. Off the coast of northeast Japan, the Subpolar Gyre (Oyashio Current) meets head-on with the Subtropical Gyre (Kuroshio Current) (Tomczak and Godfrey, 1994). Distribution of the living calcareous nannoplankton is controlled by the water masses (e.g., Okada and Honjo, 1973a), and the stratigraphic variation in the Quaternary nannoflora reflects the environmental change in surface water (e.g., Chinzei et al., 1987). Therefore, detailed investigation of the nannofossil assemblages at the surface water frontal zones is important in monitoring Quaternary paleoceanography. Here, we present a data set of Quaternary calcareous nannofossil assemblages in the subpolar frontal zone of the western Pacific Ocean that supercedes the preliminary results described by Shipboard Scientific Party (2000a, 2000b).