The study evaluated physico-chemical parameters of Kainji lake, Nigeria. Water samples were collected from three sampling stations at Kainji hydropower reservoir from July, 2013 to April, 2014. Water samples were analysed for various physico-chemical parameters using various HACH equipment. Results of the physico-chemical analysis were subjected to simple trend plot and statistical analyses descriptive using Microsoft Excel and analysis of variance (ANOVA) test in Statistical Package for Social Science (SPSS) version 16.0 to determine if there are significant difference between water quality parameters measured at upstream and downstream ends of the reservoir at 0.05 level of significance. Results of the simple trend revealed that all the parameters varied with time at upstream and downstream ends. Results also revealed that there were significant differences in the water quality parameters such as hardness, sulphate (SO42-), copper (Cu2+), nitrite (NO2-), nitrate (NO3-), manganese (Mn2+) and resistivity while parameters such as water temperature, pH, salinity, suspended solids (SS), chloride (Cl-), iron (Fe2+), chromium (Cr3+), electrical conductivity (EC), phosphate (PO43-), silicon oxide (SiO2), total dissolved solid (TDS), turbidity and dissolved oxygen (DO) were not significantly different at 0.05 level of significant at upstream and downstream locations. Comparison between the measure parameters with the Nigeria and WHO water quality standards revealed that all the parameters are within the limit permissible by the standards except NO3- concentration. High concentration of nutrients such as NO3- and PO43- may be caused by socioeconomic activities of people living around the lake. In conclusion, Kainji dam hydropower reservoir operation does not have any negative significant effect on the water quality at the station.