This study aimed to reveal the vertical and seasonal fluctuations in water quality of Keban Dam Reservoir, which is the second largest dam of Euphrates Basin in Turkey. Temperature, dissolved oxygen, pH, electrical conductivity, some cations and anions content were monthly monitored in water column between 0 and 30 meters over a twelve-month period from Febru-ary 2010 to January 2011. The thermal stratification was observed between June and August. Epilimnion extended up to depth of 10 meters and hypolimnion occurred below a depth of 20 meters. A decrease was recorded for dissolved oxygen below the epilimnion whereas for cal-cium and bicarbonate in the epilimnion during thermal stratification. The other cations and ani-ons were nearly uniform in water column. The study showed significant relations of physico-chemical variables in water column of the reservoir.