The aim of this research is to evaluate the water quality of some parts of Basement Complex and sedimentary terrain of Plateau state using water quality index (WQI) method. For this purpose, seventeen sampling sites were utilized. Seven from the Basement Complex and ten from sedimentary terrain. The samples were analyzed for various physic-chemical parameters such as pH, temperature, electrical conductivity, total dissolved solids, hardness, alkalinity, sulphate, chloride, nitrate, bicarbonate, magnesium, potassium, sodium, calcium and iron. The relative weight assigned to each parameter has a range from 1 to 5, predicated on the important parameters for drinking purposes. The computed water quality index for the fifteen samples has a range from 21.47 to 46.77. The analysis revealed that, groundwater samples from Basement Complex and Sedimentary terrain are fit for consumption, ranging from good (7 samples) to excellent (3 samples) conditions.