Abstract: The history of Arctic water masses over the past 50 ka was reconstructed by fossil ostracods from the sediment core ARC3-P31 (59 cm long under 435 m of water depth) taken from the Chukchi Plateau, Western Arctic Ocean. The ostracod assemblages of the core are predominated by the species of the three genera Krithe, Polycope and Cytheropteron, with Rabilimis mirabilis, Microcythere medistriatum, Cluthia australis, Pedicythere neofluitans and Pseudocythere caudate sporadically occurring. There is a distinct faunal turnover before and after the Last Deglaciation (LD); before the LD the ostracods are dominated by Cytheropteron-Polycope assemblages (the two genera account for 81% of the total ostracod individuals); while after the LD Polycope decreased sharply and followed by a pronounced increasing in Krithe, and Cytheropteron-Krithe combination became a feature of the assemblages (the two genera make up about 79% individually). The blooming of Krithe after the LD can be ascribed to increasing water depth of Chukchi Plateau in Holocene. The predominance of Polycope before the LD, on the other hand, is most likely due to the emergence of Arctic Intermediate Water (AIW) to shallower depths and hence the absence of warm Atlantic Water (AW) in the Last Glacial. Our results imply a possibly complete absence of AW on Chukchi Plateau during the Last Glacial and the spreading of AW into this area since Holocene.