This study was carried out between February and November 2007 in order to determine the present situation of phytoplankton and environmental factors in Lake Kaftar. Sampling was conducted seasonally in 5 different stations. Three phytoplankton and three water samples (for physicochemical analyses) were taken in each station. Phytoplankton samples were taken using 30 μm mesh plankton net and, after fixing in 4% formaldehyde, identified and counted using Nicon E200 microscope with Normarski interference optics. Water temperature, pH, dissolved oxygen, nitrate, phosphate, EC, hardness and salinity were measured by using thermometer, electrometrically performance, Winklers method, ultraviolet spectrophotometric screening method, vanadomolybdophosphoric acid colorimetric method, conductivity meter and titration methods, respectively. Highest densities of phytoplankton were recorded in Autumn and highest rates of Shannon (H` = 2.392) and Simpson (λ = 0.27) indices were seen in Spring in station 5 and 4, respectively. Totally, 18 genera belong to 10 different families and 5 classes of phytoplankton were identified in this study. Highest density of phytoplankton belongs to Chlorophyceae (38.46%), Cyanophyceae (23.08%), Zygnemophyceae (15.38%), Bacillariophyceae (15.38%) and Fragilariophyceae (7.69%), respectively. According to the results of this study, the most important environmental factors affecting diversity and density of phytoplankton in Kaftar Lake were nutrient (especially nitrate) and water temperature. Although, usually Spring phytoplankton bloom is higher than the Autumn bloom, in Kaftar Lake highest densities of phytoplankton were recorded in Autumn.