The algae of GarGar river (one of branches of Karoon River) which were investigated in samples collected from habitats (epilithic, epiphytic, epipelic) at eleven station between December 2004 and September 2005.This river receive types of sewage such as municipal wastewater, agriculture and aquaculture wastewater The flora consisted of 5 class and 24 genera belonging to the Bacillariophycea (12genera), Cyanophycea (6genera), Chlorophycea (10genera), Xanthophycea (1genera), Dinophycea (1genera). The Bacillariophycea was the dominant class and nitzschia was the most frequent genera in the epilitic substrate. The Cyaophycea was the dominant class and phormidium was the most frequent genera in the epiphytic substrate. The Bacillariophycea was the dominant from of algal flora. nitzschia, spirogyra , phormidium were the most frequent genera in the algal community. While the maximum density of the epilitic community was 282390 cell/cm2 in August and epiphytic community was 167435 cell/cm2 September. Results showed that the Algal growth is dependent on nutrienta, high temperature and slow velocity of river. The algal increased considerably from August until September. Thus, use of Algal communities is useful indicative for monitoring and assess changes in the water quality, generally changes due to anthropogenic causes (e.g. nutrients).