To tune CH4 selectivity of Fe-based Fischer-Tropsch synthesis (FTS) in the initial stage is of prime scientific and industrial importance to further improve the catalyst performance. Herein, distribution of CH4 selectivity on the metallic Fe nanoparticle is predicted by OFT calculations and micro-kinetics analysis about the competition between C-1 hydrogenations and C-1 + C-1 couplings on abundant Fe surfaces including Fe(1 0 0), Fe(1 1 0), Fe(1 1 1), Fe(2 1 1), and Fe(3 1 0). The results show that HCO mechanism (HCO -> CH + O) is an available source of C-1 species apart from CO direct dissociation. These Fe surfaces exhibit high effective barriers for CH4 formation, which is linearly correlated to the thermal stability of CH2 species. However, carbon chain prolongation on the more stable surfaces greatly depends on the coupling of C and CH species. On the less stable Fe(1 1 1) surface, the CO + C coupling is the main route for chain prolongation. Utilizing the effective barrier difference between the CH4 formation and the most feasible C-1 + C-1 coupling as a descriptor of CH4 selectivity, it is quantified that CH4 selectivity decreases in sequence of Fe(1 0 0) > Fe(2 1 1) > Fe(1 1 0) > Fe(3 1 0) > Fe(1 1 1). It is revealed that thermal stability of the CH2 species and exposition of the Fe facets could play essential roles in tuning CH4 selectivity. Trying to expand the area of Fe(2 1 1), Fe(3 1 0) and especially Fe(1 1 1) surfaces would greatly suppress CH4 selectivity without a decrease of activity. This work provides new insights and design principles for the Fe-based FTS catalysts. (C) 2019 Elsevier Inc. All rights reserved.