Abstarct: Tireh River is one of the main branches of the Dez River in Lorestan Province )Western Iran). The goal of this study is to evaluate the water quality and assess the contamination status of the Tireh River. To this end, 14 samples of the river water along the Tireh River are collected. The samples are analyzed by means of standard procedures for determination of major anion and cation and some heavy mextals (Cu, Ni, Zn, Cr, Cd, Pb and As). The obtained result shows that the Tireh River water is greatly affected by local geological rock units exposed in the study area. The major ions concentrations show large variations along the river. These fluctuations are mainly due to the lithological characteristics of the study area and some extent to inputs from anthropic sources (as for station no. 12 which is influenced by municipal effluents from Dorood). The most type of the water samples are calcic bicarbonate and their pH values ranged from 7.89 to 8.9. Also, the majority of the water samples are within the hard and alkaline category. It is assumed that hydrochemical processes such as dissolution and ion exchange anions are responsible for anion and variation in the river water. Calculating the saturation index (SI) indicated that water samples tend to become saturation relative to calcite, dolomite and aragonite and undersaturation to gypsum, halite and anhydrite. According to Wilcox diagram the river water is generally found suitable for the agricultural purposes. Regarding heavy mextals, the obtained results showed that except for the Arsenic, other mextals have low concentrations as compared to WHO standard limit. According to the statistical analysis methods such as correlation, Cluster Analysis and Principle Component Analysis, it was found that Cu, Ni, Zn, Cr and Cd has a same (natural) sources whereas Pb and As are inherited probably from anthropogenic sources.