This papers presents results of variation in species structure-composition, cell density of freshwater microalgae and the nutritional state in Dak Minh reservoir, Dak Lak province, in 2006-2007. A total of 150 species/varieties belonging to 5 phyla were reported including 53 Chlorophytes, 43 Euglenophytes, 27 Heterokontophytes, 14 Dinoflagellates, 13 Cyanophytes. 42 species/varieties were new records to freshwater microalgae flora in Vietnam. Cell density greatly varied in time and space (rainy season, surface layer: 1.133.000 cells/L, bottom layer: 781.000 cells/L
dry season, surface layer: 765.525 celis/L, bottom layer: 574.850 cells/L). In studied time, Dak Minh reservoir is the rich nutritional state.