Abstract: In supercritical water (SCW) condition, the gasification of biomass to H₂ or CH₄ has been studied extensively and proves to be critical in realising the upgrading of carbonaceous fuels. Given the extreme conditions of water at high pressure and temperature, along with the complex structure of biomass, the development of such a process still remains a challenge. In order to realize the complete decomposition of biomass and a high yield of desired products, such as CH₄ and H₂ at relatively milder conditions, various catalysts were synthesized and practiced. Different metals (such as Cr, Ni, Zn, Ru and Rh) were incorporated into various supports, such as mineral compounds of Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO, Y₂O₃, CeO₂, silica-alumina, zeolites and carbon based supports of carbon nanotube, activated carbon. As a result, the long term stability of catalys is critical in the gasification of carbonaceous fuel in supercritical water condition. Therefore, this work focused on the stability of various support materials and deactivation of active metal components in supercritical water conditions with the purpose of choosing robust catalysts. In supercritical water condition, the effect of catalyst on carbonaceous fuels cracking, methanation reaction and water gas shift reaction determines the gasification efficienty as a whole. Unfortunately, the mechanism of methanation reaction is still unclear. Therefore, the CH₄ formation mechanism and the effect of catalyst on CH₄ conversion are emphasized in this work.