In order to determine the influence of climate and hydrology on water quality of the lower Mekong River, the long term monitoring data (from 1985 to 2004) of climatic, hydrological and water quality variables were analyzed. In general, water quality was 'good' or 'very good' for most of the investigated water quality parameters including DO, pH, conductivity, nitrate, phosphate and total phosphorus. All climatic and hydrological elements as well as most of the water quality parameters varied seasonally. Throughout the 18-year period, only evaporation, water level and TSS showed a significant pertinent trend. ARIMA models results reveal that among climatic and hydrological paremeters, water quality could be effectively predicted from the data of discharge flow and precipitation. The results showed good R(2) (>/Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0.7) estimation between predicted and observed values for TSS, alkalinity and conductivity which are the chemically and biologically conservative parameters. For other water quality parameters such as Ca(2) (+ ), Mg(2)( +), Si, Cl, NO(3), and SO(2), the predicting results by ARIMA model were reliable in shorter period than the above three mentioned variables.