The performance of a high-temperature gas-cooled reactor as a tritium production device was examined. A gas turbine high-temperature reactor of 300 MWe nominal capacity (GTHTR300) was assumed as the calculation target of a typical gas-cooled reactor, and using the continuous-energy Monte Carlo transport code MVP-BURN, burn-up simulations for the entire-core region of GTHTR300 were carried out considering its unique double heterogeneity structure. It was shown that gas-cooled reactors with thermal output power of 3 GW in all can produce 6~10 kg of tritium in a year.