Abstarct: Water ecosystem including rivers is one of the components of the earth. It is considered not only as an important settlement for aquatic organisms but also as an important source of food for human beings. It is new taken for granted that the quality of water is as much important as its quantity. the term water quality is also used in physical, chemical and biological explanation for special purposes. Water quality studies have become increasingly important due to considerable decrease in water resources. However, measuring water quality parameters is highly expensive. Thus, estimating some parameters from other already measure parameters is of crucial importance. In this study, GEP model was developed to estimate water quality parameters of Tajan and Babolrood Rivers in Mazandaran. Therefore, hydrometric data on these two rivers during 1971-2011 was collected and examined. To implement GEP model, the proposed models were first provided using SPSS and stepwise regression, and then using gene Xprotools, they were examined baxsed on R2, RMSE and MAE. The results of implementing GEP model in testing and training phases showed that GEP model is of high capability in estimating TDS and SAR of Tajan and and Babolrood Rivers, offering precise mathematical relationships.