A B S T R A C TMany parameters and indicators exist for the qualitative assessment of groundwater, but their importance differs according to the plain and aquifer; therefore, evaluating the appropriate parameters for quality analysis of different areas is essential. As the main objective, the most important parameters for assessment of the quality of groundwater was selected and ranked in present study using the analytical hierarchy process. The study was carried out in the Tajan plain in northern Iran. To achieve the objectives of the research, initial information was collected, a research hierarchy was formed, and a group of experts was set up. Questionnaires were distributed to the experts and the results were recorded using Expert Choice software. The decisions from the experts were combined and the final weight of each parameter was calculated to allow selection of the most important parameters. Six graphs were plotted; five ranked the criteria, anions, cations, physical factors, and microbial factors and one ranked all parameters. At the end, the quality parameters were categorized based on their final weight from most important to least important and the six parameters of highest priority (sulfate, iron, nitrate, EC, calcium, TDS) were chosen as indicators of groundwater quality in Tajan plain. It is noteworthy that the weight of each indicator parameter was higher than the average weight of all parameters.