Lakes are dynamic and stable aquatic ecosystems that are very important both ecologically and economically. Plankton is a very important group of aquatic organisms which are the basis of food web in the planktonic food pyramid. One of the important ecological studies are temporal and spatial variations of plankton in the lakes. This study purposed to study plankton community structure from Zayandeh Rud Dam Lake, Isfahan, with emphasize on species composition, abundance and biodiversity of phytoplankton and zooplankton communities, and also their relationship with some important water quality parameters. The water and plankton sampling were carried from 9 defined-station during spring and summer 2011(with 45 days sampling intervals). Water samples were collected by Nansen water bottle sampler. Phytoplankton samples were taken by water collection from surface layer( 30 cm). Zooplankton samples were collected by using plankton net (mesh size of 50 µm) by vertical towing. The range of dissolved oxygen, temperature, EC, pH, NO 3 , PO 4 and Secchi depth of water were 7.1-11.2 mg/l, 6.0-24.8 o C, 241-361 µs/cm, 7.77-8.53, 1.36-2.31 mg/l, 1.4-39 µg/l and 0.1-8.5 m, respectively. The phytoplankton community comprised of Bacillariophyceae, Chlorophyceae, Cyanophyceae, Euglenophyceae, Dinophyceae, Cryptophyceae and Chrysophyceae with density range of 160.4-604.5, 41.4-185.9, 2.1-9.1, 0.8-17.7, 0.3-98.1, 0-27.4 and 0-23.3 cell/ml, respectively. Phytoplankton were dominated by genera of Cyclotella , Stephanodiscus , Fragilaria , Chlorella and Carteria during study period. The zooplankton community comprised of Rotiferia, Cladocera, Copepoda, Ostracoda and Nematoda with density range of 2-475, 112-214, 13-495, 0-10 and 0-4 individuals/m 3 , respectively. The Simpson, Shannon-Wiener and Margalef indices for phytoplankton were 0.67, 2.16 and 2.68 in May; 0.54, 1.95 and 1.97 in June; 0.81, 3.22 and 3.1 in August; 0.67, 2.8 and 3.97 in September, respectively. Correspondingly the Simpson, Shannon-Wiener, and Margalef indices for zooplankton were 0.83, 2.91 and 2.44 in May; 0.61, 1.84 and 1.85 in June; 0.73, 2.37 and 2.14 in August; 0.73, 2.28 and 1.43 in September, respectively. There were significant correlation between phytoplankton abundance with oxygen saturation level (r = 0.36, P = 0.03), NO 3 (r = - 0.51, P = 0.001), PO 4 (r = - 0.40, P = 0.01) and EC (r = -0.5, P = 0.01). Based on Principal Component Analysis (PCA), the most important factors of Zayandeh Rud Dam Lake which could be described plankton community stracture were pH, Secchi depth, temperature, NO 3 and PO 4 , respectively. According to findings of this research on plankton community structure, it is not recommended to stocking fish larvae at high density due to lower planktonic production levels in this lake. In addition, based on water quality and phytoplankton community, the Zayandeh Rud Dam Lake has been Keywords: Phytoplankton, Zooplankton, Abundance, Diversity, Zayandeh Rud Dam Lake, Isfahan