The influence of water contamination in the working gas on the performance of closed cycle disk MHD gen-erator was investigated experimentally. The experiment was carried out using the Fuji-1 blow-down facility. It was found that the enthalpy extraction ratio of the disk MHD generator decreased with the increase of water concentration when it was higher than about 500ppm. This concentration is remarkably higher than the permissible level predicted by previous numerical simulations. It is observed that higher seed fraction is required for higher water concentration in working gas. It is suggested that the water moleculars react with the Cs seeds in a generator. The reduction of water concentration can be realized by the increase in the bottom temperature of heat exchanger under its careful heat-up operation.