Understanding the changes in groundwater quality has an important role on the planning and sustainable management of water and soil resources in any region. In this study, trends of groundwater quality of mazandaran  plain using the information of the 24  piezometric wells were analyzed during 1994– 2011. For this purpose, the non – parametric Mann – Kendall and Spearman method were used. Before conducting the MK test, the effect of significant lag – 1 serial correlation was removed from the data set. Hydro - geochemical variables that analyzed were the total Anions, total Cations, pH, EC,TDS, SAR,  which were measured once  in a year, wet and dry periods. The magnitudes of trends were computed using the Sen’s estimator method.  The results showed that the line slope of the density changes of groundwater qualitative variables was downward in 43 percent of variables and upward in 57 percent of them. The results of our tests showed that Kendall and Spearman were positive trend related to variable EC (the equivalent of 56/3 = Z) and Mtghyrkl dissolved solids (TDS) with statistic Z (7.14), respectively. The results were also indicative of the increase in chemical density and the decrease in quality of the well water in Mazandaran plain at all piezometric stations.