This study was conducted to remove the humic acid which was well known as precursor of THMs(Trihalromethane) in chlorination at the drinking water treatment plant. In this study, humic acid could be produced a various by-products in chlorination including the chloroform which was identified with the carcinogenic compound. with the combination of ozone oxidation and AC(activated carbon) adsorption, TOC and THMFP were removed 59% and 40%, respectively.