A study on phytoplankton community was carried out from June, 1992 to May, 1993 at selected stations in Hoedong Reservoir. The phytoplankton are identified as 176 taxa including 5 phylum, 67 genera. The important species in this reservoir were Asterionella formosa, C. meneghiniana, Dictyasphaerium pulchellum, Fragilaria crotonensis, Melosira distans, M. granulata, M. granulata var. angustissima, M. granulata var, angustissima f. spiralis, Micractiniurn pussillum, Microcystis aeruginosa, Pandorina morum, Pediastrum boryanum, P. duplex, Peridinium sp., Scenedesmus quadricauda, Synedra acus, S. rumpens and S. ulna. The causative species of water bloom were identified as Microcystis aeruginosa, Trachellomonas hispida, Ceratium hirundinella, Peridinium sp., Melosira italica, Staurastrum dorsidentiferum var. ornatum in the area. During the study periods standing crops of phytoplankton were maximum in August, 1992 and minimum in December, 1992. The species dominance index and diversity index were ranged 24.7-99.9, 0.001-3.06, respectively.