Nanosized carbon black (NCB) was employed as catalyst for methane decomposition to produce hydrogen in a fluidized-bed reactor. The carbon atoms of the surface defects of NCB act as active sites in this reaction. The activity of NCB is improved after more defects in the surface of NCB are generated after the treatment in nitric acid and calcination in nitrogen gas. The loading of small amounts of Ni and Co can obviously increase the initial activity of NCB, however, their activity deceases very quickly after the reaction begins due to the encapsulation of the corresponding metal particles inside amorphous carbon produced from methane decomposition. After reaction, the formed carbon was found to grow into carbon flakes and cover the surface of NCB. The investigation with TEM and SEM indicates that they may form from a new carbon crystallite, not build upon the existing hexagon layer in the surface defects of NCB. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.