This study was carried out to analyze characteristics of water quality and correlation among water quality parameters in Idong reservoir used for agricultural water. The pH and concentrations of DO, SS, BOD, COD, TP, Chl-a in Idong reservoir almost met the water quality standards for agricultural water, but TN concentration exceeded the water quality standards for agricultural water. All of water quality items had no significant difference between horizontal points at 95% confidence level and the Idong reservoir was possible to be treated as the single water body. As NIP ratio of the Idong reservoir is 52, TP is a limiting nutrient salt. As TN exceeds the water quality standards, it is required to preserve TP below current state to prevent water bloom by eutrophication. Therefore, reduction of phosphorus from the watershed is necessary for controlling the eutrophication of Idong reservoir.