Reservoir dam serves as ecologically important resource to aquaculture and are socioeconomically important. As dam construction develops, such reservoirs studies are began on plankton, benthos and fish to investigate ways to increase fish production in the lakes. Zooplankton diversity is found as one of the most important ecological parameters for link between fish and phytoplankton, and plays a key role in organic matter cycle in aquatic ecosystems. Shian Dam is located adjacent to Islam Abad city in coordinates 34° 4´ N and 46° 41´ E. In this study, given lake area and limitations, three sampling stations were considered and samples were taken in 4 phases throughout the year. Evaluation of physicochemical parameters showed no significant differences between the different stations, but significant difference was found between seasons. Temperature ranged 7-20 °C, pH = 8-8.9, EC ranged 1267-1562 ms/cm, and ammonia 0.13- 0.133mg/l, phosphate ranged 8.9-143.3 mg/l and soluble solids 636.32-776.66 mg/ l in different seasons. Zooplankton samples were caught by plankton net with mesh in 140 microns and were fixed with 4% formalin. The main zooplankton found in Shian lake included two genera Bosmina sp. and Simocephalus sp. from Ceriodaphnia with a density 0-720.4 ind./m³, Copepods include Copepodit، Naupli Copepoda and Diaptomus sp. with density 165.7-6101.2 ind./m³ and rotifers as Ascomorpha sp. ، Asplanchna sp. ، Polyarthra sp.، Keratella sp، Brachionus sp. in density 506.7-4257.3ind./m³. The range of Shannon-Wiener's index was estimated 1.08-1.96 , Simpson's index 0.43-0.71 and Margalef's index was 3.76-5.6. High density of Brachionus sp. And Keratella sp. are as indicator of polluted waters. Also high physicochemical parameters as phosphate and EC denote on lake water pollution and non-suitability for domestic consumption. Keywords: Mesozooplankton, Shian dam lake, Shannon-Wiener's index , Margalef's index, Simpson's index, Kermanshah
