The FTS reactions over Ru/Co/Al2O3 catalyst were performed in the shell and tube type fixed-bed reactor with an oil circulation system in order to investigate the effect of N-2 and CO2 content. It was found that the N-2 gas does not affect the product selectivity of the FTS reaction over Ru/Co/Al2O3 catalyst in the fixed-bed reactor at pilot scale but it has decreased the CO conversion. It was also found that the CO2 has affected the product selectivity of the FTS reaction through the chain growth reaction. It eventually has led the product distribution to shift to the short chain hydrocarbons in FTS reaction. As a result it is concluded that removing the impurities from the syngas in commercial GTL and GTL-FPSO plants is essential to enhance the economic efficiency of process.