Since cyanobacteria are the base of life and productivity of aquatic ecosystems, sustainable ecological study of the Caspian Sea, particularly the distribution and identification of species composition, density and biomass, seasonal and regional variations in cyanobacteria before each study seems necessary.Plankton is strongly affected by numerous pesticides and herbicides. Synthetic detergents are very toxic for marine life, and water pollution with this agent affect the microbial flora. Marine life is easily affected, but the most deaths are due to lack of oxygen in the polluted water, and dumping of pesticides and toxic residue.Therefore, the purpose of this work was to study cyanobacteria of different seasons.For maturing of cyanobacteria Factors, choose 6 line Lisar, Anzaly, Sefidrood, Nooshahr, Babolsar and Amirabad that every line have 4 stations(A, B, C, D) and water for analysis have taken from different deeps ( 0,5,10,20,50,100) meter in southern of Caspian sea and then transferred to laboratory of Caspian sea ecological institute. Then the samples transferred to laboratory of Ecological Academy, kept in cool and darkness in properly capped glass bottles. The cyanobacteria was analyzed on a “Nikon” light microscope at ×480 magnification. Algae abundance was determined using the Hydro bios counting chamber and sampled (volume 0.1 ml).The volume of each cell was then calculated by measuring its appropriate morph metric characteristics and geometric. Finally, the volume values were converted to biomass. In this study, cell abundance and biomass of in different seasons were significant different. The dominant cell abundance was recorded in spring while the minimum were in winter. Shannon-Weaver diversity index (Shannon et al., 1963) of species ranges from 0.00 to 0.696 that belong to Oscillatoria tenuis.Ecological and environmental alterations may trigger changes in the cyanobacteria species number, abundance and biomass. Then, since cyanobacteria depend upon certain conditions for growth, they are a good indicator of change in their environment. For these reasons, and because they also exert a globalscale influence on climate, cyanobacteria are of primary interest to oceanographers and Earth scientists around the world. Algae and produce the bulk of primary production in seas. During this study, Cyabobacteria community in the Caspian Sea changed in different years and the maximum cell abundance and biomass observed in summer and Shannon-Weaver diversity index changed in this study.