theta-Fe3C has been reported to play a key role in C-2-C-4 olefins production for Fischer-Tropsch synthesis. In this work, (031) is confirmed as the most exposed facet of theta-Fe3C due to its thermodynamic stability. Investigation on CH4 formation and C-1-C-1 coupling reveals that CH4 formation exhibits a high effective barrier and CH + CH and CH2 + CH2 are the dominant chain growth pathways. Delta E-eff was employed to quantify the selectivity of CH4 and C2+. The high value of DEeff indicates the preference of C2+ formation to CH4. These results will help for the design of selective Fe-based FT catalysts. (C) 2017 Elsevier B.V. All rights reserved.