The present invention relates to a novel biocide composition for use in gravity fed water purification devices and suitable for purifying water for drinking purposes and a process for preparing the novel biocide composition. The biocide tablet composition comprises one or more halogenated 5,5-dialkylhydantoin compound； a chlorine biocide compound selected from trichloroisocyanuric acid (TCCA) or sodium dichloroisocyanurate (NaDCC) or mixtures thereof； 0.1-1.0% water soluble boron containing compound； wherein the ratio between the halogenated 5,5-dialkylhydantoin compound and chlorine biocide compound is in the range 85:15 to 65:35 and the average particle size of the halogenated 5,5-dialkylhydantoin compound and chlorine biocide compound is in the range 400 to1200 microns.
