The major trouble in the RO method of seawater desalination is biological fouling (biofouling) by microorganism multiplication. By carrying Out direct clilorine sterilization of the RO membrane with an Intermittent chlorine injection (ICI) method, biofouling trouble is eliminated. In this work, change of characteristics of a cellulose triacetate (CTA) membrane in the case of chlorine injection to seawater containing heavy metals was established quantitatively by experiments and development of a performance prediction formula for the CTA membrane. In addition, the validity of the performance prediction formula was verified by comparing with the actual performance change in the desalination plants. Finally, optimization of ICI condition was attained by using this newly established prediction formula.