Some of the cities in Kurdistan Region of Iraq depend completely on the groundwater, as a main source of water in their water supply systems, where the cities are too far from surface water resources, and because the mountainous topography of the land makes the transformation of surface water for long distances, too difficult. One of those towns, which depends on the groundwater, as one of the main sources in water supply is Koi Sanjak, located in Erbil Governorate, north of Iraq.In this study, the groundwater is carefully investigated by evaluating the performance of water wells, which are located within Haibat Sultan Mountain; to the east of the city. From the previous geological investigations, the study area includes four geological formations, which are from oldest to the youngest, Kolosh, Gercus, Pila Spi and Fatha formations. The deep wells are drilled within Pila Spi Formation, the lithology consists of fissured marly limestone, thickness of the aquifer ranges between (149 – 188) m, this type of formation is considered as a good aquifer for the groundwater accumulation. For the study of the hydraulic characteristics of the aquifer, five deep wells were selected. The single well test method was used for analyzing the well characteristics; since observation wells are not available in the studied area. Pumping test data are analyzed to obtain the hydraulic properties of the aquifer, such as transmissivity, coefficient of permeability, specific capacity and specific yield. The pumping test also gives information about hydro-geological properties of the aquifer, such as the type and thickness of the aquifer.The previous geological investigation in Haibat Sultan Mountain exhibits, that Haibat Sultan aquifer is of unconfined type; while the results of the current study exhibit that the aquifer varies from unconfined, at some location to leaky aquifer in others. This may give an expectation that the slope sediments on the overlying layer may act as aquitard and causing the aquifer to behave as a leaky aquifer. The current total yield of Haibat Sultan aquifer is 1194 m3/day.