For advanced trihalomethanes control, the removal of humic acid, which is one of trihalomethanes precursors, was investigated. The smaller the particle size of activated carbon, the more humic acid was adsorbed. It will be presumed that the contact surface area dominates the amount of humic acid adsorbed. Moreover, the humic acid solution was treated with ozone, and the adsorption removal by activated carbon was investigated. The amount of humic acid adsorbed increased with ozonization time. It is assumed that degradation to lower molecular weight fragments by ozonolysis contributed to the increase in the amount adsorbed onto activated carbon. It was concluded that preozonation was effective in removing humic acid.