Quaternary ammonium compounds, Couple, Cotton fiber, Antibacterial activity. Abstract. a gemini-type quaternary ammonium compound with a hydroxyl group in their spacer chain, 4,5-bis(dodecyl dimethyl ammonium bromide)-1-pentanol (called as DAP12), was immobilized on the surface of cotton fiber by using a silane coupler, tetraethoxysilane and a novel antibacterial cotton fiber (called as CF12) was prepared. Antibacterial characteristics of CF12 were investigated comparatively with [3-(trimethoxysilyl) propyl] octadecyldimethylammonium chloride(TPOAC), a mono-type quaternary ammonium salt. CF12 had excellent antibacterial characteristics in the wide range of environmental pH and temperature, which is not weakened even if 30 times repetitive use. The experimental results indicated also that CF12 has higher antimicrobial activity than TPOAC immobilized on the surface of cotton fiber against a broad range of microorganisms, and average inhibition ratio against 15 strains of gram-negative, gram-positive bacteria and yeasts is 99.0% at 30 °C and pH 6.0.