Phytoplankton samples, collected about twenty day interval on May of 1967, at same twenty-one stations located in the north-eastern Korea Strait were examined in an attempt to determine the specific distribution and abundance of phytoplankton. Water bottle samples preserved with neutralized formula were used for the phytoplankton study. In May, water temperature and salinity show both lower at the sea of Korea side than that of Japan side, and both indicate gradually higher to the sea of Japan side. Fourty-six species of diatoms and three species of dinoflagellates in early May, and thirty-six species of diatoms and six species of dinoflagellates in the May were detected. Among them thirty species of diatoms and two species of dinoflagellates were appeared commonly in both cruise. Chaetoceros curvisetus and Rhizosolenia castracanei were wide distributed species, and Leptocylindrus danicus was the most numerous species at this sea in May. In generally, the sea of Korea side was more rich in species and numbers of phytoplankton.