In the present study the freshwater algae of Siahdarvishan River were sampled on a montly basis from December 2007 through November 2008. Three sampling stations (Gaght roudkhan , Shahkezr dame and Talab laleh ) were chosen in the river. Some physical and chemical parameters (water temperature , total hardness , nitrate , nitrite, phosphate and pH) of water samples were measured. A total of 84 species were identified including: Chlorophyta, 36 species; Cyanophyta, 24 species; Bacillariophyta, 20 species; Rhodophyta, 2 species; Euglenophyta and Xanthophyta each with 1 species. Among the families, Oscillatoriaceae with 14 species showed the highest species richness. Taxonomic keys to genera and species are presented to facilitate the identification of algal flora of the region.