The present work is aimed at assessing the water quality index (WQI) for the ground water of Koilwar block. This has been determined by collecting ground water samples to a comprehensive physico chemical analysis. In the present study Sixty water samples(six water samples from one village) are taken from ten different villages of Koilwar block and 11 water quality parameters are have been considered: Temperature, pH, Total hardness, Iron, Chloride, Fluoride and Total dissolved solids, Calcium, Magnesium, Sulphate and alkalinity. The WQI for these samples ranges from 40.67- 69.59. The analysis reveals that the ground water of the Koilwar block needs some treatment before consumption and it is also needs to be protected from contamination.