Abstarct: The main aim of this study is evaluation the effect of the operation of the Ghaleh-Chay Dam on the quantity and quality of groundwater in Ajabshir Plain. Ajabshir Plain, with an area of approximately 120 square kilometers, is located in the South East of Tabriz. This plain is a part of Urumiyeh Lake catchment area and one of the most important agricultural regions of East Azarbaijan province that is at risk of water crisis. During 20 years (1360 to 1380) the level of groundwater has dropped about 6 meters due to excessive withdrawal of groundwater and it led to reduce the groundwater quantity and quality. Ghale-Chay Dam has been constructed on the Ghaleh-Chay River in the upper part of the plain, which has been operated since 1388. The studies in this research indicated that the level of ground water in most parts of plain was increased after the dam opration. Such that after dam opration the average slope of groundwater level has been increased from -0.04 to 0.05. However, in a number of piezometers, the average slope of groundwater level has not been changed or slightly decreased. Discharge of springs and qanats is reduced significantly after the operation of Ghale-Chay Dam, these are located in the coarse grained alluvial adjacent to the river and due to the reduced flow of the Ghale-Chay River after the dam constraction, decreased discharge of them. Because groundwater quality is directly related to the quantity of water resource, in the upper part of plain the electrical conductivity of ground water dose not have a significant change compared to the years before the operation of dam. But at the lower part of plain electrical conductivity of groundwater has increased after the dam operation. In this way, before the dam's operation the electrical conductivity in the lower part of plains is about 4200 µS/cm and after dam operation has grown to about 4800 µS/cm. As well the electrical conductivity of springs and qanats have decreased.