In this paper, we examed the removal effect of intermediate-chlorination on THMs to compare with pre-chlorination and investigated to apply intermediate-chlorination to real water treatment plant. Additionally, we investigated the mechanism of THMs causes of IOM+SOM which is organic matter of algae. Chlorine dose of Kanchang was reduced 10% by coagulation. As a result, 30% of THMs was reduced and 20% of THMs was reduced at IOM+SOM experiment.