Rivers as one of the most important water resources are very vulnerable to the human activites. Development of reservoirs and dams is one of the oldest forms of the human interferences in the aquatic ecosystems. Despite many benefits of dams (hydroelectric power, flood control and water level regulation), they have also the potential of change in the population of aquatic organisms. Construction of dams causes significant changes in the flow regime, traort of suspended particle, channel morphology, temperature of water, and chemical changes especially in the downstream of rivers. Macroinvertebrates are affected by changes in the habitat. The effect of dams on the macroinvertebrate communities is very important because of their role in the river ecosystem function. Zayaderud River is one of the largest and the most important rivers in the central plateau of Iran that its flow regime is affected by Zayanderud dam. In order to assess the ecological impact of Zayanderud dam construction on the biological communities as well as the water quality, 6 stations (Kharsoonak and Organ stations in upstream of the dam, Chadegan station in a distance from near the below of the dam, Hojjatabaad, Maarkade, and Hoore stations in downstream of the dam) were selected and the water quality and macroinvertebrates have been investigated in a quantitative method from Jun, 2013 to May, 2014. Sampling of water and macroinvertebrates was done in each station with 3 replications every 45-day (Two times in each season). Water quality parameters including temperature, dissolved oxygen, nitrate, phosphate, BOD 5 , COD, EC and pH were measured by standard methods. Biological indices including taxonomic richness, BMWP, ASPT, and Shannon, Simpson, and Margalef diversity were calculated for macroinvertebrate communities. The results showed that the chemical and physical properties of water (except temperature) between the regions of upstream and downstream was not affected by the dam structure and was more influenced by seasonal changes. Also biodiversity indicators including richness, Shannon, Simpson, and Margalef indexes showed a significant reduction especially in the station below the dam and can be affected by the dam structure. Density of many taxones showed a significant reduction in the dam station. In addition to the density and the type of taxson were reduced in the downstream stations of the dam and were replaced by the taxson compatible with the new conditions. Also BMWP and ASPT indices showed that the water quality in the station below the dam is similar to polluted, probably polluted to medium polluted, respectively. This quality decrease is due to the change in the density and diversity of macroinvertebrate, not to the organic pollution. Keywords: dam, Zayanderud, biological indicators, macro invertebrates, water quality