Highly ordered mesoporous bimodal structures of KIT-6 with average pore diameters of 3.6 and 5.4 nm originated from its different wall thickness were newly synthesized and applied for CO hydrogenation to hydrocarbons through Fischer-Tropsch Synthesis (FTS) reaction using the supported iron nanoparticles. The ordered bimodal mesopore structures of the KIT-6 with smaller than 5.4 nm in size showed a significantly enhanced FTS activity by selectively forming heterogeneous bimodal crystallite size distributions of iron nanoparticles. The positive effects of the mesoporous bimodal structures of the KIT-6 were mainly attributed to the facile formation of the spatially confined smaller iron nanoparticles, which can be strongly interacted with the structurally stable ordered mesoporous KIT-6 with the co-presence of the larger iron nanoparticles on the outer surfaces of the mesoporous bimodal KIT-6. After adding small amount of potassium promoter with 2 wt% K on the mesoporous bimodal Fe/KIT-6, a slight increase of CO conversion as well as C-5+ selectivity with less extent of CO2 formation was observed due to the suppressed activity of water-gas shift reaction. However, the structurally unstable monomodal KIT-6 having a relatively larger mesopore above 7.3 nm showed a lower FTS activity by preferentially forming an inactive amorphous carbon species. A superior FTS activity on the ordered mesoporous bimodal Fe/KIT-6 was attributed to the easy formations of smaller iron carbides, where the active smaller iron carbides were strongly interacted and spatially confined inside of the highly ordered mesoporous bimodal KIT-6 surfaces.