This paper presents the effects of acid treatment on the activity, product selectivity and life span of iron Fischer–Tropsch (FT) catalysts supported on carbon nanotubes (CNTs). Two different types of CNTs with low surface area (∼25m²/g) and high surface area (∼170m²/g) were prepared and treated with 35wt% HNO₃ at 25°C and 110°C for 16h. Fe/CNTs were prepared using incipient wetness impregnation method with iron loading of 10wt%. The CNT supports and catalysts were characterized by nitrogen adsorption, ICP, TPR, XRD, SEM, TEM and Raman spectroscopy. The acid treatments at 25°C and 110°C increased the BET surface area by 18% and 31%. The results of Raman analysis revealed that the acid treatment increased the number of defects which are considered as anchoring site for metal particles. TEM analysis showed that the major parts of the iron particles were homogenously distributed inside the acid treated nanotubes. According to the XRD analysis, the acid treatment on both families of CNTs resulted in decrease in metal particle sizes. The FT synthesis was carried out in a fixed-bed microreactor (275°C, 2MPa, CO/H₂=2) for 120h. Among the catalysts studied, Fe catalyst supported on pre-treated CNTs at 110°C was stable and active while, the other catalysts experienced rapid deactivations. The Fe catalyst supported on CNTs low surface area and larger diameter showed much lower CH₄ and higher C₅⁺ selectivities.