The species composition and numerical abundance of copepods related to tides, seasons and stations were investigated in Chiku lagoon and coastal waters of Tainan from August 1998 to June 1999. The grazing impact on phytoplankton by copepods in the study area was measured and the species composition and numerical abundance of copepods collected by two nets with different mashes (330&#x00B5;m and 100 &#x00B5;m) were compared and discussed. In total 72 species of copepods belonging to 14 families and 4 orders (42 Calanoids, 2 Harpacticoids, 19 Poecilostomatoids, and 8 Cyclopoids) were identified in the samples collected by 330&#x00B5;m mash net in the Chiku lagoon. The mean abundance of copepods was 1.5¡Ó3.5 ¡Ñ104 ind./100m3. The most dominant groups were Parvocalanus crasirostris, Oithona simplex, Oithona rigida and unidentified Copepodite, comprised 68 % of the total copepod numbers. The highest abundance of copepods was observed in October and February. Higher abundance and species number were found at station I than at station M, and were found more during high tide than at low tide. Thirty-eight species of the copepods belonging to 13 familis and 4 olders. (22 Calanoids, 2 Harpacticoids, 8 Poecilostomatoids and 6 Cyclopoids) were found in the samples collected by 100£gm mash net, with mean abundance of 1.1¡Ó1.9 ¡Ñ106 ind./100m3. The most dominant species were mostly small-sized and immature copepods, such as Parvocalanus crassirostris ¡BCopepodite¡BOithona rigida¡BOithona brevicornis ¡BOithona nana¡BNauplii¡BOithona simplex¡BEuterpina acutifrons and Acrocalanus gracilis, and comprised 90 % of the total numbers. Abundance and species number of copoeds varied seasonally, with highest abundance and species number found in February. The abundance of copoeds at M station was found higher than that at I station. Sixty-two species of the Copepod belonging to 13 families and 4 orders (36 Calanoids, 2 Harpacticoids, 20 Poecilostomatoids and 4 Cycloids) were found in the coastal watersof tainan, with mean abundance of 6.2 ¡Ó7.9 ¡Ñ104 ind./100m3. The most dominant copoepods in this area were warm-water and neritic species, such as Farranula gibbula, Acrocalanus gibber, Temora discaudata, A. gracilis, O. atlantica, Copepodite and T. turbinata, and comprised 62 % of the total catch. The abundance was significantly different among station, highest abundance found at station 3, that is the closest station to Chiku lagoon. The grazing impact of copepods on phytoplankton was minor, only < 2 % / day.