Pilot tests were performed to investigate the effectiveness of ozonation for the treatment of drinking water. Prior to the experiment, four regional target water were analyzed for determining target compounds. Various organics including pesticides were identified and present also in the conventionally treated tap water. The contamination of pesticides in most raw water was severe and ozonation was found to be effective to remove pesticides significantly. Other organic species were removed effectively as the order of aromatic amines, nitro compounds, ketones and ethers. On the whole, volatile organics were removed effectively than DOC, CH2Cl2 extractable organics. Accompanying with the increasement of AOC, aldehydes have increased after ozonation and reduced by post-GAC. Also, bromate was produced after ozonation and it was validated to suppress the production of bromate on the presence of ammonia and DOC.