The comprehensive study of high-temperature gas-cooled reactors (HTR) has been carried out for many years. In particular, the Chinese 10 MW(t) high-temperature gas-cooled reactor test module (HTR-10) has attained initial criticality since December 20, 2000. The HTR-10 design represents the features of modular HTGR design. The reactor core and steam generator are housed in two steel pressure vessels, which are arranged side by side. These steel pressure vessels are in touch with cold helium of about 250degreesC coming out from the circulator which sits over the steam generator tubes in the same vessel. A feasibility study for a four-module 200 MW(t) HTR, generating about 300 MW of electricity, has recently been performed by INET, Tsinghua University. This paper presents the economic feasibility of a seawater desalination system for HTR by reverse osmosis (RO) (HTR-300), that produces 480, 000m(3)/d of fresh water, which was calculated using the Desalination Economic Evaluation Program (DEEP2.0) developed by the IAEA. The results indicated that the averaged discounted production of water cost is $0.57/m(3) and the averaged discounted electricity generation cost is $0.038 /kWh, which is lower than that of a nuclear and fossil power plant for desalination plants. It has the advantages of its low electricity cost and water production cost for its inherent safety features of HTR, compared to other same-scale nuclear power and fossil power plants. The sensitive factors for the cost of water desalination and electricity generation were analyzed, which include net thermal efficiency, discount rate/interest rate, specific investment cost and seawater temperature/seawater total dissolved solid (TDS). The discount rate is one of the factors that has the greatest effect on the cost of water and electricity, which are $0.42/m(3) and $0.028/kWh, $0.48/m(3) and $0.032/kWh, $0.57/m(3) and $0.038/kWh, $0.65/m(3) and $0.043/kWh, $0.73/m(3) and $0.048/kWh, respectively, while the discount rate/interest rate varies in the range of 3%, 5%, 8%, 10% and 12%. The water cost is strongly influenced by the TDS of the seawater, but not influenced by seawater temperature. The reference net thermal efficiency is another factor that has a great effect on water and electricity cost: $0.59/m(3) and $0.043/kWh at 3 5%, $0.56/m(3) and $0.034/kWh at 47.7%.