The West Pacific Ocean is considered as the provenance center of global marine life and has the highest speciesdiversity of numerous marine taxa.The phytoplankton,as the primary producer at the base of the food chain,effects on climate change,fish resources as well as the entire ecosystem.However,there are few large-scale surveys covering several currents with different hydrographic characteristics.This study aimed to explore the relationships between the spatio-temporal variation in phytoplankton community structure and different water masses.A total of 630 water samples and 90 net samples of phytoplankton were collected at 45 stations in the Northwest Pacific Ocean (21.0°–42.0°N,118.0°–156.0°E) during spring and summer 2017.A total of 281 phytoplankton taxa (>5 μm) belonging to 61 genera were identified in the study area.The distribution patt of the phytoplankton community differed significantly both spatially and temporally.The average abundances of phytoplankton in spring and summer were 797.07×102 cells/L and 84.94×102 cells/L,respectively.Whether in spring or summer,the maximum abundance always appeared in the north transition region affected by the Oyashio Current,where nutrients were abundant and diatoms dominated the phytoplankton community;whereas the phytoplankton abundance was very low in the oligotrophic Kuroshio region,and the proportion of dinoflagellates in total abundance increased significantly.The horizontal distribution of phytoplankton abundance increased from low to high latitudes,which was consistent with the trend of nutrient distributions,but contrary to that of water temperature and salinity.In the north area affected by the Oyashio Current,the phytoplankton abundance was mainly concentrated in the upper 30 m of water column,while the maximum abundance often occurred at depths of 50–75 m in the south-central area affected by the Kuroshio Current.Pearson correlation and redundancy analysis (RDA) showed that phytoplankton abundance was significant negatively correlated with temperature and salinity,but positively correlated with nutrient concentration.The phytoplankton community structure was mainly determined by nutrient availability,especially the N:P ratio.