Fischer-Tropsch synthesis in supercritical-phase was developed and its reactional behavior and mass transfer phenomenon were analyzed by both experimental and simulation methods. The same reaction apparatus and catalysts were used for both gas-phase reaction and liquid-phase F-T reactions to correctly compare characteristic features of diffusion dynamics and the reaction itself, in various reaction-phases. Efficient transportation of reactants and products to the inside of catalyst bed and pellet, quick heat transfer and in situ wax extraction from catalyst by supercritical fluid were accomplished. As an application of this process, wax could be produced effectively in the supercritical-phase reaction, especially in the case when heavy olefin was added to the supercritical fluid.