This study was carried out in order to investigate the mineralogical characteristics and paleoenvironmental changes of the core sediments (03GHP-02 and HB13-2), obtained in the Ulleung basin and Hupo basin, Korea. The results on mineral compositions, clay mineral compositions, and the total contents and sequential extraction of different fractions of the phosphorus in core samples showed that those values are different in two cores and also at different depths. In core 03GHP-02, the minerals are composed mainly of quartz, microcline, albite, calcite, opal A, pyrite, and clay minerals(illite, chlorite, kaolinite, and smectite). In 0.7-3.5 m range in depth, the low content of opal A and the low illite crystallinity can be inferred to indicate the relatively cool climate, which corresponds to the ice age. Also, the content of total phosphorus was low in those samples. It was reported that East Sea at that time was isolated from the neighboring seas due to the decrease of the sea level, and as a result, the influx of sediments was supposed to be little through the strait and rivers. In core HB13-2, the main minerals are almost the same as those from the core of Ulleung basin, but there is no noticeable changes with the core depth. There are no significant changes in clay mineral composition and the distribution of phosphorus with increasing depth either. This little change can be interpreted to indicate that the sediments comprising the core might be deposited in a relatively short period of time or deposited in sedimentary environment in which there''s no significant changes in sediment supplies. The values of crystallinity of clay minerals are high in those samples, indicating that it was relatively warm during that time. Although the increase of fluctuation pattern can be observed, showing that the climate of this period often changed, it is supposed that during the period of sediment deposition, it was generally warm.