Seasonal fluctuations in abundances and ingestion rates of the three major copepods, Acartia hongi, Calanus sinicus and Paracalanus parvus s. l., around the Taean Peninsula were studied along with the estimation of the grazing impacts by them on phytoplankton standing stocks. These three copepods occupied about 50% of total mesozooplankton abundances and about 70% of total copepod abundances. A. hongi dominated in winter and spring while C. sinicus showed only one peak in spring. P parvus s. l. occurred dominantly in summer and fall. The ingestion rates of these three copepods were the highest in spring, when their abundances of eggs and nauplii were more than 10 times greater. Abundances of copepodites of these copepods were also very abundant at this time of high ingestion rates. These increased ingestion rates seemed to be related with reproduction. The grazing impacts by these three copepods were about 5% of the available chlorophyll a in the water column (with the range of 0.7 to 40.5%). The highest value was found in spring.