Study of groundwater quality trend in any region is very important from the view of scheduling and sustainable water resources management. In the present study trend analysis conducted on groundwater quality based on information of 13 piezometeric stations located in the Ardabil plain from 1374 to 1387 using the nonparametric mann-Kendall test. The effect of significant lag-1 autocorrelation removed from the data series prior to analysis. Hydrogeochemical variables studied here are Total anions, Total Cations, pH, TH, EC, SO42-, Na%, Na+, Mg2+, K+, Ca2+, TDS, SAR, Cl- and Hco3- which are measured two times a year. The trend of line slope is estimated by Sen’s estimator method for each time series. Results showed that concentration of all groundwater quality variables exhibited upward trends at all stations. Statistical significant positive trends (p<0.05) observed for 23 percent of stations in wet month and 19.5 percent in dry month of a year. Results for slope of trend lines were shown by Box and Whisker plots. In wet month strong positive trends observed for water quality in the central and east part of the Ardabil plain. However, in wet month such positive trends observed in the central and east of the plain under study.