The phytoplankton of the Derbent Dam Lake was investigated between February 2001 and July 2002. In this study, a total 180 taxa belong to 8 division have been identified. These were Cyanophyta (Cyanobacteria) [22 taxa; 12%], Bacillariophyta [74 taxa; 41%], Chlorophyta [69 taxa; 38%], Chrysophyta [1 taxa; 1%], Cryptophyta [2 taxa; 1%], Euglenophyta [6 taxa; 3%], Pyrrophyta [3 taxa; 2%] and Xanthophyta [3 taxa; 2%]. According to planktonic algae composition, Derbent Dam Lake has a changing characteristic from oligotrophy to mesotrophy.