The algae in Dongbok reservoir consists of 6 class, 14 orders, 30 families, 63 genera, 145 species, 13 varieties, and 1 formula in 159 taxa during jan. 1995 to Dec. 1999. Species numbers according to major taxa were appeared as Chlorophyceae 80 taxa, Bacillariophyceae 28 taxa, and Cyanophyceae 17 taxa. The ratio of total standing crops were Bacillariopyceae 44%, Cyanophyceae 30%, and Chlorophyceae 225. Various species remarkably appeared during fall to early winter comparing with other seasons. Stnading crops of algae decreased from 1.1~3.7 million units/$ell$ in 1995 to below 1.0 million n\units/$ell$ ever after 1996. Species number was very variable as 9~37 in down stram comparing with 12~34 in middle stream, 15~24 in upper stream. Species diversity indices were 1.1~4.0 in 1995, 1.5~3.5 in 1996~1997, 2.1~3.6 in 1998, and 0.3~3.4 in 1999. Dominant indices were 0.35~0.97 in 1995, 0.44~0.86 in 1996, 0.43~0.89 in 1997, 0.39~0.80 in 1998, and 0.45~0.97 in 1999.