A critical study has been made on the diversity of Cyanophyceae in Haranbari Dam of Bagalan during the period from July,1998 to June,2000. Algal samples were collected at 1st week of monthly interval from three stations of Haranbari dam during the period of two years of investigation. Line drawing of Cyanophyceae, were made by camera lucida. 47 taxa of Cyanophyceae, were identified with the help of standard monograph and recent literature. In present study this class consist of mainly species of Microsystis, Chroococcus, Gloeocapsa, Aphanocapsa, Synechococus, Gomphosphaeri, Hydrococcus, Arthrospira, Spirulina, Oscillatoria, Trichodesmium, Phormidium, Lyngbya, Nostoc, Anabaena, Raphidiopsis, Homoeothrix, Calothrix, Rivularia and Gloeothrichia. The member of Nostocales were found to be dominant as compaire to other forms of Blue Green Algae (BGA). All these forms were observed from three stations of Haranbari dam. In the present study 47 taxa of belonging 20 genera are Cyanophyceae were studied texanomically.