This study covers the performance of five cobalt-based catalytic systems with different support characteristics in Fisher-Tropsch synthesis (FTS) under conventional gas-phase using syngas, and the manner in which the reaction medium is influenced by the addition of solvents such as n-hexane and squalane has been also investigated. The reaction was conducted in a fixed bed high-pressure FTS reactor setup at a reaction temperature of 220 ℃ to 260 ℃. In terms of the effect of the reaction medium, higher CO conversion was obtained in gas phase reaction, whereas the olefin selectivity was higher in n-hexane medium than in gas phase and squalane medium. In addition, the undesired production of CH4 and CO2 was relatively reduced in solvent addition compared to the gas phase FTS. The reaction performance was also compared according to the type of catalyst. CO conversion by type of catalyst decreased in the following order: 20% Co/SiO2>20% Co/Al2O3>20% Co/HAS>20% Co/Si-MMS>20% Co/TiO2. However, the C5+ content of products was little affected by catalyst supports.