Seawater and brackish desalination using reverse osmosis (RO) has been a pragmatic approach to mitigating fresh water shortage in many water-stressed areas worldwide. Compared with other desalination technologies, RO has several attributes including high energy efficiency and system modularization. To achieve these notable attributes, for large-scale seawater RO plants, it is critical to accurately design and evaluate the RO process before plant installation and operation. In this study, the IMSDesign software is applied to design a seawater RO plant with a capacity of 100 m3 of fresh water per hour. The software allows to customize the design parameters and operating conditions of the RO plant to meet the design requirements. After inputting design parameters and operating conditions, the RO plant operation can be simulated to obtain its performance indexes for plant design evaluation. The evaluation results reveal that the RO plant with membrane trains arranged in 2-passes configuration can obtain high quality permeate that meets the required standards for drinking water at energy consumption and water cost of 10.66 kWh/m3 and 2.48 USD/m3, respectively.