Addition of a small amount of middle olefins into a supercritical-phase Fischer-Tropsch reaction medium significantly promoted the carbon-chain growth and greatly enhanced the selectivity of waxy products, with increased CO conversion while successfully suppressing methane and CO2 selectivity. Hydrocarbon products exhibited anti-ASF distribution. The influences of reaction conditions and catalyst pore size, were studied for the supercritical-phase FT reaction where middle olefin was co-fed. This phenomenon was not observed in the gas-phase reaction and liquid-phase reaction conducted in a trickle bed. But liquid-phase reaction conducted in batch type reactor exhibited the same olefin addition effect. The reaction performances mentioned above were analyzed from the viewpoint of mass transfer efficiency of the added olefin in various reaction mediums.