Display omitted •Reduction of Mo-Fe/MgO catalyst leads to high-yield carbon production.•Increasing reduction temperature leads to larger CNT diameter and higher crystallinity.•Increasing reduction hydrogen partial pressure (pH2) leads to larger CNT diameter.•Optimum reduction conditions (Temp, pH2, Time) of Mo-Fe/MgO catalyst for high quality CNT were found. The effects of temperature, hydrogen partial pressure, and time in catalytic reduction step on carbon nanotube growth in a catalytic methane decomposition have been investigated for Mo-Fe/MgO catalysts. The results show that the reduction conditions of the catalyst affect the crystal structure of the metal formed on the catalyst surface and the growth mechanism of the generated carbon nanotubes. Both diameter distribution and crystallinity of the CNTs increased with the increase of reduction temperature in the range of 400 to 800 °C. The optimal reduction temperature with the maximum carbon yield was found to be 500 °C. The increase of hydrogen partial pressure and reduction time increased the CNT diameter distribution, and the optimal hydrogen partial pressure and reduction time with maximum carbon yield were found to be 0.1 atm, 60 min and 0.3 atm, 5 min, respectively. In the different combination of hydrogen partial pressure and reduction time for maximizing carbon yield, the CNT average diameter did not have a significant change, while the CNT crystallinity showed opposite trends depending on the methane decomposition reaction time. Ultimately, it was confirmed that the Mo-Fe/MgO catalyst can change the properties of CNTs produced through control of reduction conditions.