This chapter describes the enhancement of reaction rate and inhibition of coking on the catalyst using supercritical fluid and elucidates the solvent effect on the reaction. Supercritical fluid is a fluid above its critical temperature and pressure. Around the critical point, the fluid properties vary greatly with a little change of pressure and temperature. Quite a few research and development for the supercritical fluid extraction process have been reported by using fine control of the solvent power and selectivity of extraction. The solvent effect on reaction equilibrium or reaction rate can be manipulated due to its controllable fluid properties. Supercritical fluid is employed as a reaction media of a new coal conversion process. The chapter discusses the specific features of the supercritical water extraction of coal and elucidates the role of water on the reaction. For catalytic reforming of coal tar pitch, supercritical toluene–tetralin mixture is used.