Abstarct: Groundwater is the most important resource for drinking, agriculture and industry in Islamshahr city. In this study, in order to investigate the factors affecting the quality of groundwater sources in Islamshahr, 21 water samples were collected and analyzed for the concentration of major ions, nitrate, and potentially toxic elements. The concentrations of major ions and nitrate were measured by ion chromatography (IC), and the values of potentially toxic elements (including arsenic, cadmium, copper, iron, manganese, nickel, lead, and zinc) were measured using an ICP-MS instrument. pH, total dissolved solids (TDS), and electrical conductivity (EC) were measured at the field. The average concentration of sodium, calcium, magnesium, and potassium cations in water samples are 172.6, 96.7, 28.2, and 1.2 mg/L, respectively.The average concentrations of bicarbonate, sulfate, and chloride are 278.4, 113.8 and 332.3 mg/L, respectively. The average concentration of nitrate in the studied samples is 228 mg/L, and in 70% of the samples, the nitrate concentration is higher than the international and national standard values. The Piper diagram shows that the studied water samples are mainly Na-HCO3, which reflects the geology of the aquifer (alluvial formations). Moreover, the values of ionic ratios indicate that evaporation and dissolution are the main factors controlling the hydrochemical characteristics of the water samples. baxsed on Wilcox's diagram, most water samples are classified as C3-S4. Therefore, the groundwater of the study area is not suitable for irrigation. The water quality indices show that the quality of studied groundwater varies from suitable to very poor for drinking usage. baxsed on the values of residual sodium carbonate (RSC), the studied samples are not suitable for irrigation. The Langelier coefficient also indicates that the samples are not suitable for industrial application. The average concentration of arsenic, cadmium, copper, iron, manganese, nickel, lead and zinc is 2.06, 0.2, 1.22, 121.03, 4.82, 3.61, 0.32 and 36.82 µg/L. Except for Cd in some sampling sites, the concentrations of all studied elements are within the permissible limit for drinking. On the basis of Contamination Degrre and Heavy mextal Pollution Index (HPI) values, some studied stations are heavily polluted with heavy mextals.