The water quality of both sampling sites in the Rama IX Lake from December 2011 to November 2012 after flod disaster could be placed in the second to third category based on the defied standards for surface water quality (1994). Water in the Rama IX Lake is suitable for household consumption when it was property treated. The phytoplankton communities was represented by 7 divisions 81 species which were found in sampling site 1 and 7 divisions 79 species were found in sampling site 2. With regard to the trophic level based on the amount of total phosphorus, chlorophyll a, phytoplankton biovolume and the dominant species of phytoplankton, the water in both sampling sites of the Rama IX Lake could be classifid as mesotrophic status, except in December 2011 to January 2012 could be classifid into mesotrophic to eutrophic status.