Fischer-Tropsch synthesis is the technology of converting a syngas (CO+H2) derived from such as coal, natural gas and biomass into a hydrocarbon using a catalyst. The catalyst used in the Fischer Tropsch synthesis consists of active metal, promoter and support. The types of these components and composition affect the reaction activity and product selectivity. In this study, we manufactured an iron catalyst using γ-Al2O3/SiO2 mixed support (100/0 wt%, 75/25 wt%, 50/ 50 wt%, 25/75 wt%, 0/100 wt%) by an impregnation method to investigate how the composition of γ-Al2O3/SiO2 mixed support effects on the reaction activity and product selectivity. The physical properties of catalyst were analyzed by N2 physical adsorption and X-Ray diffraction method. The Fischer-Tropsch synthesis was conducted at 300 °C, 20bar in a fixed bed reactor for 60h. According to the results of the N2 physical adsorption analysis, the BET surface area decreases as the composition of γ-Al2O3 decreases, and the pore volume and pore average diameter increase as the composition of γ- Al2O3 decreases except for the composition of γ-Al2O3/SiO2 of 50/50 wt%. By the results of the X-Ray diffraction analysis, the particle size of α-Fe2O3 decreases as the composition of γ-Al2O3 decreases. As a result of the Fischer-Tropsch synthesis, the CO conversion decreases as the composition of γ-Al2O3 decreases, and the selectivity of C1-C4 decreases until the composition of γ-Al2O3 was 25 wt%. In contrast, the selectivity of C5+ increases until the composition of γ Al2O3 is 25 wt%.