In this paper, a simple methodology based on multivariate analysis is developed to create a groundwater quality index (GWQI). The methodology is based on three general stages, selection, standardization and aggregation, to develop a groundwater quality index in an area using regional and local standards. In this method, through selecting a set of similar parameters, standard values of the parameters are obtained by dividing the averaged concentration of parameters by their maximum allowable concentration cited in Iran’s Water Quality Standard. Final indices for ground water quality are derived through weighting the standard magnitudes of the parameters. In order to appraise the approach, existing water quality data for the ground water of the Qazvin plateau were used. In the selection phase, eight important parameters (K+ , Na+, Ca2+, Mg2+, SO2-4, Cl-, TDS, pH) for the potability of ground water were selected. These parameters were more accurately monitored and documented in the study area and are very important in desirable drinking water. In order to identify the suitable range of indices, the derived indices were compared with those obtained for some mineral waters. Sketching a comparative ground water iso-index map in the study area makes it possible to have a comprehensive and easily interpretable picture of the ground water quality in the area of Qazvin plateau. Results show the proximity of the derived indices to those obtained for mineral water in some regions of Qazvin Province. It is clear that evaluating the overall suitability of the ground water for drinking usage in an area essentially needs survey of other parameters in addition to those observed here.