Phytoplankton play an important role as primary producer in aquatic ecosystem and a number of factors have been attributed to influence the density of it. The present study was conducted to assess the relationship between the physico – chemical parameters and phytoplankton density of Kangsabati Reservoir. A monthly sampling was carried out from March, 2010 – February, 2011 at three different stations. The eight classes of phytoplankton comprises 64 species among which Chlorophyceae 21 species, Bacillariophyceae 14 species, Cyanophyceae 12 species, Charophyceae 8 species, Tribophyceae 2 species, Dinophyceae 4 genera, Ulvophyceae 1 species and Xanthophyceae 2 species,. The phytoplankton population density showed significant correlation with the parameters like temperature, D.O., phosphate, total inorganic nitrogen etc. This population was high in winter months and low in the month of rainy season.