Seawater temperature, salinity, dissolved oxygen, nutrients and chlorophyll-a have been studied around Hamduck in the northern part of Cheju Island during July, 1989-July, 1990. In the surface water ranges of water temperature, salinity, dissolved oxygen, nitrate, phospate, silicate and chlorophyll-a have been 13.7-26.5℃, 28.51-34.37‰, 4.47-7.14㎖/ℓ, ND-15.42㎍-at/ℓ, ND-1.47㎍-at/ℓ, 2.33-24.1㎍-at/ℓ, and 0.02-2.24㎎/㎥, respectively. Yearly mean values of the near surface waters(0m-10m)at station 6 show high temperature, nitrate, chlorophyll-a, while the values of dissolved oxygen, phosphate, and silicate were low compared with the other stations. The vertical profiles are likely to show the possibility of upwelling in October and tidal front in July, suggesting high productivity of phytoplankton around St.6. N1P ratio varies from 35 inshore to 15 offshore and from 45 in July to 15 in Ferbruary. These variation patterns are similar to those of water temperature.