An attempt has been made to study the phytoplankton diversity of a freshwater, seasonal oxbow lake, Baishar Beel of Nadia district of West Bengal. This particular beel is located within Chakdaha block of Nadia district and shows a connection between rural and urban areas. A total of 30 phytoplankton species were recorded from the wetland during the study period of March-November, 2012. Phytoplankton composition of the Baishar Beel was fluctuated by seasonal changes and four algal classes namely Chlorophyceae, Cyanophyceae, Bacillariophyceae and Euglenophyceae form the phytoplankton spectrum within the study period. Among this, Chlorophyceae dominates in monsoon and post-monsoon and Cyanophyceae dominates in pre-monsoon season. Various diversity indices (Shannon Wiener diversity index, Margalef species richness index, Pielou evenness index, and Simpson dominance index) were calculated to illustrate the seasonal changes of phytoplankton. The Shannon Wiener diversity index values (2.52, 2.33 and 2.66 for pre-monsoon, monsoon and post-monsoon season respectively) were recorded. Which pinpoint a moderate pollution status of this beel?