Supercritical-phase Fischer-Tropsch synthesis co-fed with 1-tetradecene was carried out over Co/SiO2 catalysts. It was found that a supercritical fluid could extract the product from the catalyst bed efficiently, and as a result, the mass transfers for both reactants and products were enhanced, 1-Tetradecene added as a chain initiator could participate in the chain growth process. This consequently increased the rate of formation of hydrocarbons larger than C-14 Significantly, and decreased the yield of C-1-C-13 hydrocarbons, leading to a flatter carbon number distribution of product than that obtained without olefin addition. In addition, wax composition analysis also showed that the average molecular weight and degree of saturation of wax increased due to addition of 1-tetradecene. The effects of catalyst pore size, concentration of co-fed 1-olefin, and CO/H-2 ratio of syngas were investigated.