A series of 29 sampling with a 330 ${mu}$m and a 64 ${mu}$m mesh size of nets was conducted at a fixed station in Ilkwang Bay, southeast cost of Korea, from Oct, 2, 1991 to Oct. 10, 1992, to investigate the effects of mesh size of nets on biomass estimation of copepod Acartia steueri. The catch of copepodite and nauplius stages of A. steueli taken by two nets with different mesh size was different, showing that all developmental stages of A. steueri were retained on the 64 ${mu}$m mesh net, but only $geq$stage 4 copepodite were caught by the 330 ${mu}$m mesh net. Abundance and biomass in each developmental stage estimated with the 64 ${mu}$m mesh net were significantly higher than those of the 330 ${mu}$m mesh net, except for adult female and stage 5 copepodite in female. The body length as well as the body width is likely to affect the catch of the nets. The mean biomass of A. steueli estimated with the traditional 330 ${mu}$m net was 2.8 times lower than the value obtained with the 64 ${mu}$m mesh net. However, the seasonal patterns of the biomass were comparable. These results suggest that accurate sampling strategr of the entire copepods assemblage including nauplii and copepodites are essential when estimating the abundance and biomass of copepods for the better understanding of the role of copepods in marine ecosystem.