A biological assessment of water quality was carried out in the Pyungchang River water system, Gangwon-do, Korea from September 1999 through August 2000. The benthic macroinvertebrates collected seasonally from 15 field sites were identified mostly up to species level. Benthic macroinvertebrates composed of 109 species, 77 genera, 31 families, 10 orders, 4 classes in 4 phyla. Dominant species occurring in all the field sites were mostly ephemeropterans and trichopterans. On the basis of the values of speices diversity and biological indices such as Trent Biotic Index(TBI) Modified Biological Monitoring Working Party Score System(BMWP) and Biotic Index(BI), the Pyungchang River Water system was generally regarded as showing oligosaprobic water quality.