Pre-treating the multi-walled carbon nanotubes (CNTs) support by refluxing in 35 vol nitric acid followed by heating at the temperature of 600 to 900 °C resulted in the formation of defects on the CNTs. Increasing the temperature of the pre-treatment of the CNTs from 600 °C to 900 °C, enhanced the fraction of cobalt-oxide nanoparticles encapsulated in the channels of CNTs from 31 to 70. The performance of Co/CNTs in Fischer-Tropsch synthesis (FTS) was evaluated in a fixed-bed micro-reactor at a temperature of 240 °C and a pressure of 2.0 MPa. The highest CO conversion obtained over Co/CNTs. A.900 was 59 and it dropped by ~3 after 130 h of time-on-stream. However, maximum CO conversion using Co/CNTs. A.600 catalysts was 28 and it decreased rapidly by about 54 after 130 h of time-on-stream. These findings show that the combined acid and thermal pre-treatment of CNTs support at 900 °C has improved the stability and activity of the Co/CNTs catalyst in FTS. © 2018 by the authors.