The synthesized hydrocarbons through CO hydrogenation of syngas have the environmentally benign characteristics such as a sulfur-free and low content of aromatics. Ordered mesoporous bimodal structures of KIT-6 were newly synthesized, and iron nanoparticles were deposited on the surface. The bimodal structures enhanced FTS activity by selectively forming heterogeneous bimodal crystallite size distributions of iron nanoparticles. It was due to the facile formation of the spatially confined smaller iron nanoparticles with the stronger interaction. As the small amount of potassium promoter with 2wt% was added, CO conversion and C₅₊ selectivity were slightly increased due to the suppressed activity of water-gas shift reaction.