The seasonal variation of phytoplankton of TajBaj Khan Pokhra, Hajipur, Vaishali, Bihar in relation to different abiotic and biotic factor were studied. The major constituent of phytoplankton was Chlorophyceae (60.72%) followed by Cyanophyceae (29.55%) and Bacillariophyceae (9.69%). Each group showed different peaks period and was found to be related with duration of sunshine, temperature, turbidity, pH, D.O. and free carbon dioxide content of the pond water.