The phytoplankton diversity of river Budhi Gandak of Khagaria (Bihar) in relation to certain physico-chemical factors was studied. A total of 82 genera of phytoplankton, belonging to four groups – chlorophyceae, cyanophyceae, bacillariophycae and euglenophyceae were found in the river water. The phytoplankton density in different seasons was in order of winter > summer > monsoon. The species belonging to bacillariophyceae were dominant. A significant positive correlation was established in between total density of phytoplankton and various physico- chemical factors such as TDS and conductivity. The species diversity of phytoplankton (H¯) varied from 0.3 to 2.8, the relative abundance of algal species indicate Navicula subtilissima, Melosira granulata and Synedra ulna as indicator of the pollution of the river. On the basis Shannon Weiner index river water was moderate to heavily polluted condition. WQI were in the range of poor category in monsoon season almost at all the three sites.