To prevent formation of trihalomethanes (THMs) in drinking water, removing precursors of trihalomethanes (PTHMs) in water resources for tap water is essential. We compared the following three treatments for removal of PTHMs: activated carbon (AC), ozone (OZ) and ozone-activated carbon combination (OZAC). Orange II (OR, an acidic dye), methylene blue (MB, a basic dye) and humic acid (HA) were used as PTHMs. HA exists abundantly as PTHM in nature. Results demonstrated that PTHMs could be decomposed or removed by either AC or OZ treatment. Efficiency of removal of HA by the three treatment methods was lower than that for removal of OR and MB, as the molecules of HA were larger than those of OR and MB. Decreases of total organic carbon values were achieved by treatment of MB with OZ or HA with AC. As for advanced water treatment, a two-step sequential process of OZ-AC treatments is currently used. However, the present results demonstrated that highly efficient removal of PTHMs could be accomplished by the addition of AC during OZ treatment.