For working out the countermeasure against the water pollution at Yedang Reservoir in Yesan-gun, Chung Nam Province, thirty factors of water properties were checked seasonally in 1991. The values of TSS, turbidity, DO, BOD, COD, Cl^-, NH_4^+ and NO_2^- were 8.0-20.7mg/l, 3.7-25.0, 9.8-12.7mg/l, 1.95-5.25mg/l, 5.2-14.Omg/l, 3.4-11.3mg/l, 0.270㎍/l and 5-60㎍/l respectively, and the highest in summer. But contents of total phosphate(24-5l㎍/l), ABS(13.2-64.0㎍/l) and chlorophyll-a (2.0-42.7mg/l) were higher in autumn than any other season. The contents of CN^- and toxic heavy metals in water were zero or low annually. The eutrophication of Yedang reservoir could be classified to mesotrophic lake from summer to fall season and oligotrophic-mesotrophic lake from winter to spring season by measuring amount of total nitrogen, total phosphorus and chlorophyll-a. On the Environmental Quality Standard of Lake, water qualities of Yedang Reservoir were 3-5 levels, but contents of toxic heavy metals and synthetic substance did not exceed the level of Human Health Safeguard.