The limnological survey of Lake Okjong was conducted for one year from June 1993 to May 1994 on the monthly basis. The loading of phosphorus, nitrogen and organic carbon from the watershed into the lake were monitored at the main inflowing sites. Secchi disc transparency, epilimnetic chlorophyll a, total nitrogen, total phosphorus concentration and primary production were in the range of 1.3∼4.0 m, 2.4∼18.7 mg Chl/m^3, 1.25∼2.87 mgN/l, 7∼65 mg P/m^3, 325∼2,113 mg C/m^2/day, respectively. TN/TP atomic ratio varied from 129 to 443. N/P ratio decreased in summer because phosphorus concentration was higher than in winter, while nitrogen did not vary much. The seasonal succession of phytoplankton was distinct. In winter and spring, diatoms, Aulacoseira granulata var. angustissima and Aulacoseira italica were dominant while cyanobacteria, Microcystis sp., M.ichthyoblabe, Phormidium sp. and P. valderianum var. tenuis were dominant in warm seasons. The dominant zooplankton species were Thermocyclops taihokuensis in warm seasons while Bosmina longirostris were dominant in cold seasons. The organic carbon, nitrogen and phosphorus content of lake sediment were 8.0∼14.8, 0.59∼0.71, 1.14∼1.87mg N/g, respectively. The sediment of Lake Okjong can be classified as oligohumic based on C/N ratio. The total phosphorus loading from the watershed and fishfarm were estimated to be 2.7 g P/m^2/yr, which far exceeded the critical loading for eutrophication. The organic carbon loading from the watershed and primary production were determined to be 998t C/yr, 6,348t C/yr, respectively. Most of organic carbon was contributed by autochthonous primary production of phytoplankton. Trophic state of Lake Okjong can be classified as eutrophic.