Seasonal changes in the diversity of phytoplankton and its relationships to the water quality in Büyükcekmece Watershed in Istanbul, Turkey were studied in this study during one-year period, from January 2009 to December 2009. Physico-chemical parameters, main criteria of water quality, were observed. Having identified species from 6 divisions, including Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta, Dinophyta and Cryptophyta within the phytoplankton of Büyükcekmece Lake, and that of the named influent streams, Bacillariophyta among these divisions has been specified as the richest group in species variety. Among the streams having gone under sampling, Tahtakoprü Stream has displayed the species variety (48), and respectively Karasu (45), Hamza Stream (32), Beylikcayiri Stream (21) and Ahlat Stream (7) have come thereafter. Within the phytoplankton of Büyükcekmece Lake, 66 species have been identified. It was determined that the water qualities of Büyükcekmece Lake and related streams were varies I, II and III.