In order to estimate a better methodological factor to understand phytoplankton ecology between abundance and bio volume of phytoplankton, each 1,160 phytoplankton data, including abundance, classification and chlorophyll a concentration were collected in Korean coastal waters of Incheon (Yellow sea), Tongyeong (South sea), and Ulsan (East sea). Based on these data, phytoplankton bio volume can be calculated through a geometric model. The correlation coefficient between abundance and chlorophyll a concentration was higher than the coefficient between biovolume and chlorophyll a concentration, because a small size phytoplankton has relatively dense chlorophyll contents compared with the proportion of chlorophyll in a large size phytoplankton. Thus, the interpretation using abundance to understand phytoplankton ecology in Korean coastal waters may be more effective than that using bio volume.