To produce isoparaffins directly from synthesis gas, the mixture of CO and H-2, modified Fischer-Tropsch (FT) synthesis was carried out using a hybrid catalyst consisting Of CO/SiO2 and HZSM-5 under supercritical n-butane circumstances. Introduction of supercritical-phase n-butane increased light isoparaffins significantly and suppressed the formation of the byproduct, methane. Under supercritical-phase butane, hydrogenolysis and isomerization reactions were promoted. It is considered that because of high solubility of supercritical-phase n-butane, heavy hydrocarbons and water, the byproduct from FT synthesis, were removed quickly from acidic sites of zeolite; thus, hydrogenolysis and isomerization reactions of hydrocarbons were promoted.