Zooplanktons have an important role in food chain of the aquatic ecosystems, which acts as a food chain link between producers (phytoplankton) and higher nutritional levels. The purpose of this research was to identify and study the zooplankton density in the southeastern basin of the Caspian Sea (Gorgan Bay). Sampling was performed seasonally and samples were collected by zooplankton net (36 cm and mesh size of 100µm) with three replications from three areas including: pan-cultures, Ashooradeh pier and entrance of sea water to the Bay. In this study, zooplankton groups belong to 16 species of 6 phylum were identified containing: Copepoda, Rotatoria, Protozoa, intermediate zooplankton (zoobenthose) and a group including other zooplankton groups. The mean Shannon diversity index during sampling time was 1.57±0.26, Margalef species richness index was 1.83±0.43, and for the uniformity index was 0.69±0.12. The lowest Shannon Diversity Index was 1.12 in the autumn station 6 was considered due to human activity, commuting boats (tourists and fish hunting), and the oil pollutants in this region, and the highest Margalef index was 2.76 in the spring station 2 and the Pielou's evenness index was highest for the winter at station 5 (0.92). Cluster analysis and non-metric multidimensional scaling showed the similarity of stations with each other based on abundance distribution of species in two distinct groups. Finding of research could play an important role in environmental protection in the Gorgan Bay.