An experimental study was conducted to investigate the effects of chlorine dioxide and ozone on reduction of THM(trihalomethane) formation. Precursor concentration, chlorine concentration, reaction time, pH, and temperature were governing compornents of THM formation. When other conditions are constant, THM formation increased linearly with precursor concentration increased. THM formation increased when pH increased from 5 to 9. In combined treatment with chlorine and chlorine dioxide, chlorine treatment after chlorine dioxide treatment made less THM than any other case does. Ozonation reduced THMFP(THM formation potential) of THM precursor. THMFP decreased exponentially with reaction time increased. Also biodegradability of humic acid was enhanced by ozonation.