One of the most critical issues, we are facing today, in RO system is bio-fouling. At the previous IDA conference held on November, 2009 at Dubai, we have introduced a new slime control agent as the countermeasure. We have been using this new slime control agent in many different commercial RO plants throughout the world, and it subsequently has been proven to be quite effective. This time, we discuss some case studies and advantageous effects for RO system.We have verified that this new slime control agent is quite effective to the commercial RO systems in Japanese semiconductor plants, and a Chinese seawater desalination pant. Since the RO feed water had a very high potential of slime growth and bio-fouling, the customer faced with series of problems, such as increased differential pressure across RO membranes, decreased flux and salt rejection. Moreover, customer had frequently repeated RO cleaning in order to resolve the problems. We started to use the new slime control agent, having a characteristic of peeling-off ability to bio-fouling, to the RO plant. As a result, problems described above were all improved greatly; differential pressure was improved, the decrease of flux was inhibited, and salt rejection was improved and maintained. In addition, the interval of RO cleaning was stretched. Since the quality of RO permeate stably improved, the amount of de-ionized water obtained at the ion-exchange system after RO units were increased. Furthermore, we performed RO membrane autopsy and verified a significant decrease of bio-fouling on the RO membrane with the use of the new slime control agent.