Abstract Background and purpose:In many regions especially in the north of Iran groundwater is the most important resource for drinking supply. A present study was carried out in the Tajan plain in northern Iran. The aim of this study was to evaluate the quality of groundwater. Materials and Methods:In order to constructing the groundwater quality index (GWQI) 3 main levels were composed selection, standardization, and aggregation.The analytical hierarchy process (AHP) as a powerful multi-criteria decision-making approach was used to select the indicator parameter and determining the weights,the national drinking water standard of Iran was considered forstandardization level, finally the qualityindex values in each well were calculated by aggregating the sub-index of entire parameters. For analyzing the spatial distribution of GWQI the geographic information system were applied, the status of groundwater quality in Tajan plain was interpolated on a map. Results:The results showed that the “GWQI” values varied between 0.145 and 0.450,according to this range four quality classes were determined on interpolated map. Analyses showed that 27.8% of the study area has very good quality aquifer, 57.8% has good quality, 10.9% of Tajan aquifer has moderate quality, and 3.5% is poor. It is considerable that the quality of groundwater around the urban zone is poor. Conclusion:It was concluded in present study that the“AHP” is a reasonable method for selection of the most appropriate parameters also suitable technique for calculation of the weights for determining the GWQI.