Agriculture is the main non-point source of pollution of groundwater in irrigated areas as fertilizers and other agrochemicals are the main contaminants in the water that drains out of the root zone to recharge the aquifers. Nitrate released from fertilizers, and dissolved in percolation water, is lost from irrigated fields. The concentration of nitrate in the percolated water depends on the variation of soil mloisture in the field and nitrogen balance over the area. Its concentration in the groundwater depends on the total recharge, pollution loading, groundwater flow and solute traort within the aquifer. In this research, non-point source pollution of groundwater and its variation in Arak plain, located in Markazi province, ws investigated by applying AVSWAT-X, Visual Modflow-4 and its solute traort model MT3DMS simultaneously. From fall 2006 until summer 2007, seasonal pH, DO, NO 3 , Fe, Zn, Pb, EC, TDS, BOD, COD and Total Coliforms were measured in 40 selected wells. In addition, 42 geoelectric soundings were performed to recognize bedrock and aquifer type. Nitrate leaching was estimated by using AVSWAT-X model and its inputs including DEM, soil map, land use, climate information, fertilizers application and cultivated crops. Then, the aquifer was simulated by gathering input data such as climate data, hydrogeologic parameters, hydrology data, water level in pizometric wells and water withdrawal from the aquifer and using Visual Modflow-4 and MT3DMS subroutine. The model was calibrated with PEST code. Correlation coefficient and RMSE were 0.998 and 2.27% for groundwater flow and 0.88 and 15% for nitrate traort in groundwater in calibration stage. In general, application of these models resulted in good estimation of nitrate concentration in groundwater. Some of the wells had only nitrate and EC content higher than drinking standards. Mean nitrate concentration for the whole period was 22.92 mg/lit and it range was 5.21-53.88 mg/lit in different wells. Seasonal averages of nitrate concentration from fall 2006 till summer 2007 were 21.32, 20.49, 24.29 and 25.58 mg/lit, respectively. Generally, EC of 83% of the wells was higher than standards (0.7 dS/m) for drinking. Modeling results showed that nitrate concentration has been increasing in some regions such as suburb of Arak , and west and south of the plain. Simulation results shwed that in the next 15 years, nitrate concentration has almost doubled in the west region (from 10 to 20 mg/lit) and from 20 to 35 mg/lit in the suburb of Arak . However, it does not change in the north, but the plume has enlarged and moved to the east