Estuaries are one of the most productive aquatic ecosystems that both ecologically and economically are important. Zooplankton is a very important group of animals in estuarine ecosystems because they are the basis for food web and intermediate steps in the planktonic food pyramid. Ecological studies, especially temporal and spatial variations of zooplankton in estuaries are considerable. The present study emphasized on zooplankton from Helleh River estuary, Busheher, Persian Gulf that composition, abundance, biomass and diversity of zooplankton communities were investigated species together with some of water quality characteristics. Samples were taken by using plankton net with mesh size of 140 µm with vertical towing from 5 fixed-stations in mid-season from summer 2011 to spring 2012 for one year period. In this study, the stations were selected by environmental gradients, flow dynamics and mixing of fresh and salt water, tides, river flow rate, and geomorphology. The measurements showed that the amount of the dissolved oxygen , temprature, salinity, pH, chlorophyll a and the visible depth of Secchi disk were varied in the range of 6.45-12.25 mg/l, 13-34.4 o C , 9-45 ppt, 8.04-8.23, 0.04-0.44 mg/m 3 and 40-55 cm, respectively. The zooplankton community comprised of Arthropoda, Annelida, Protozoa, Mollusca, Echinodermata, Chordata, Rotatoria, Nematoda, Ctenophora, and Cnidaria. All samples were dominated by copepods, especially Acartia sp. In addition, Arthropoda had highest abundance while Echinodermata, Rotatoria, Nematoda, Rotatoria, and Ctenophora showed lowest abundance of Arthropoda, copepods with 10 genera including; Acartia, Euterpina, Oithona, Oncaea , Paracalanus , Corycaeus , Labidocera , Macrosetella , Microsetella , and Temora were at the highest abundance in zooplankton communities. The maximum abundance and biomass was estimated at station 5 (mouth of estuary) in all seasons, highest values in autumn (46051.9 ind./m 3 and 165.0 mg/m 3 ) and lowest in winter (5388.4 ind./m 3 and 20.9 mg/m 3 ). The biodiversity indices of Simpson, Shannon-Wiener, and Margalef were obtained 0.84, 3.01 and 1.90 in summer; 0.63, 2.20 and 1.87 in autumn; 0.73, 2.32 and 1.44 in winter; 0.87, 3.44 and 2.38 in spring, respectively. The highest zooplankton biodiversity was at station 2 during summer, at station 1 in autumn and winter and at station 5 in spring. The season similarity based on Jaccard’s and S?rensen’s coefficients were highest values of 0.61 and 0.76 for summer-autumn, 0.56 and 0.72 for autumn-spring while these value were lowest value of 0.27 and 0.43 for winter-autumn, 0.30 and 0.47 for spring-winter, respectively. In addition, there were significant correlation between zooplankton abundance with salinity (r=0.68, P 0.01), dissolved oxygen (r=-0.59, P 0.01) and chlorophyll a (r=0.71, P 0.01), biomass with salinity (r=0.67, P 0.01), dissolved oxygen (r=-0.54, P 0.01) and chlorophyll a (r=0.68, P 0.01), temperature with Simpson (r=0.44, P 0.05) and Shannon-Wiener (r= -0.51, P 0.05), and pH with Margalef (r= 0.53, P 0.05). Based on PCA(Principal Component Analysis), the most important factors in Helleh river’s estuary were chlorophyll a , salinity, temperature and pH, respectively. This study is useful in understanding the ecological and biological processes for sustainable fisheries management in estuaries. Keywords: Zooplankton, Biomass, Abundance, Diversity, Helleh River’s Estuary, Persian Gulf