An experimental and theoretical study of trihalomethanes formation in Taipei City drinking waters is presented. These researchers compared the measured values with the calculated values according to a model developed for raw water in a distribution system. Case study for measuring the concentration of trihalomethanes formation in two plants for water treatment and a water distribution systems of Taipei area were performed. Results show that the observed data were generally with 10% deviation of the simulated results. Effects of dispersion number and reaction time of trihalomethanes formation in the piping system at a fixed chlorine dosage are also discussed.