N,O-functionalized and high-temperature-treated carbon nanotubes (N, O, HT-CNTs) were employed to investigate the promotional effects of CNT on Fe catalysts for the Fischer-Tropsch-to-olefins (FTO); particularly, the origin of active sites was explored using multiple in/ex situ techniques. An upward trend in initial activity, olefin selectivity, olefin/paraffin ratio, and chain growth probability is observed in the sequence of O-, N-, and HT-CNT supported catalysts. Unexpectedly, Fe/HT-CNT exhibits superb selectivity of 72.4% to C-2-C-7 olefins, closing to the Anderson-Schulz-Flory (ASF) limit. The defect-rich electron withdrawing HT-CNT weakens the reducibility of FeOx and the C3H6 adsorption on Fe, in contrast to O-, N-CNT. The defect-anchored Fe/HT-CNT promotes "Fe-C" formation and phase transformation toward epsilon-Fe2.2C/epsilon-Fe2C, improving CO adsorption and activation. The above merits boost olefin selectivity on Fe/HT-CNT. However, O-CNT abounded with O groups retards the formation and deep carbonization of the active phase, indicating a gradual increase in the yield. (C) 2018 Elsevier Inc. All rights reserved.