Abstarct: Groundwater resources are among the most important sources of water used in agriculture and drinking, and therefore evaluating the hydrochemical characteristics of these resources is of very critical. The present study was carried out to evaluate the environmental hydrogeochemical properties of groundwater resources of Mazj-Jilan plain (Semnan province). For this purpose, 13 groundwater samples were taken from different abstraction wells throughout the plain. It was generally found that the concentration of the major ions (anions and cations) and physicochemical parameters (EC, TDS, pH) increased from the recharge zone towards the aquifer discharge along the direction flow of groundwater (except for samples near the of Kal-Shoor River that shows different trends). Regarding spatial variation of parameters and evaluation of the saturation indices of the main minerals, and also the calculation of some ionic ratios, it was revealed that dissolution of evaporative minerals, ion exchange, the existence of Kal-Shoor River and possibly mixing is the most important factor controlling the hydrochemical characteristics of the groundwater resources of Mazj-Jilan plain. baxsed on the Piper diagram, Dorov and Stiff diagrams, the hydrochemical types of water are found to be chlorine for 7 samples, sulfate for 5 samples and one sample is of bicarbonate type. Also, hydrochemical facies are found to be calcic samples for 8 water samples and sodic for 5 samples. In order to evaluate the effect of agricultural activities on groundwater quality, nitrate concentration was determined and it was revealed nitrate concentrations in all investigated wells (on average 5.82 mg/l) were below the maximum permissible level. The results of the statistical analysis of the hydrochemical data (Pearson correlation and hierarchical clustering method) showed that the parameters were divided into two groups in terms of similarity and statistical relationships: the first group consisted of sodium, calcium, magnesium, chlorine, sulfate, total hardness, EC and the second group contains bicarbonate and pH. This data grouping shows the similarity of the hydrosmithic behavior of variables of the same origin as measured variables.