A crosslinked fully aromatic polyamide ultra-thin composite membrane, designed UTC-80, has been developed for seawater desalination. The excellent membrane performance was derived from the combination of both design of membrane materials and membrane fabrication technology. We found that the trihalomethan rejection of the UTC-80 membrane was much higher even in the case of the seawater desalination compared to the cellulose acetate membrane. The affinity of trihalomethane with membrane materials is presumed to cause the difference of trihalomethan rejection. On the other hand, decreasing of salt rejection by the oxygenation agent is a serious probrem. So plant operating technology is a one of the important technology to product high quarity water continuousry by the RO seawater desalination plant.