This paper describes a catalytic reaction of hydrogen and carbon monoxide (Fischer-Tropsch synthesis (FTS)) over carbon nanotubes (CNTs) supported cobalt nanoparticles. We have investigated the effect of calcination of the catalysts on FTS performance using X-ray diffraction (XRD), H(2) chemisorption, temperature programmed reduction (TPR), temperature programmed oxidation (TPO), and transmission electron microscopy (TEM) techniques. With the increase of outer diameter of CNTs, specific surface area of the catalyst decreases while Co particle size increased accompanying with a decrease in CO conversion. The FTS performance is similar for samples calcined in N(2) or air at temperature below 550 degrees C. Over 550 degrees C, the results are much different in that the Co/CNTs can keep its activity due to the unchanged CNTs structure in N(2) while the Co/CNTs almost lose activity owing to the loss of CNTs structure and sintering of cobalt oxide clusters in air. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.