A floristic study on the diatom was carried out in coastal area at two seasons, August 2014 and April 2015, from three islands in the northern area of Yellow Sea of South Korea. A total of 117 species of diatoms, including 3 classes, 9 subclasses, 20 orders, 32 families and 54 genera, were identified based on LM and SEM examinations. Three species of diatoms, Gyrosigma obscurum, Pleurosigma frenguellianum, and P. sabangi were newly recorded in Korea. The historical synonyms of the species are presented for the floristic informations, the examined specimen for the each taxon is specified, and the distribution in three islands and detailed sampling localities of each taxon are given.