Two wet biomass gasification processes, supercritical water gasification and biomethanation, were evaluated from energy, environmental, and economic aspects. Gasification of I dry-t/d of water hyacinth was taken as a model case. Assumptions were made that system should be energetically independent. that no environmentally harmful material should be released, and that carbon dioxide should be removed from the product gas. Energy efficiency, carbon dioxide payback time, and price of the product gas were chosen as indices for energy, environmental, and economic evaluation. respectively. Under the conditions assumed here, supercritical water gasification is evaluated to be more advantageous over biomethanation, but the cost of the product gas is still 1.86 times more expensive than city gas in Tokyo. To improve efficiency of supercritical water gasification, improvement of heat exchanger efficiency, is effective. Utilization of fermentation sludge will make biomethanation much more advantageous.