The influence of thermal treatment in hydrogen and nitrogen atmosphere of Co precursors in Co/MWCNTs system on the catalyst structure and catalytic activities in NH3 decomposition were studied. The cobalt-support interaction in cobalt supported on MWCNTs was controlled by varying the temperature of thermal treatment of Co precursor and reaction media. Due to the interaction of cobalt species with multi wall carbon nanotubes (MWCNTs), the Co/MWCNTs catalysts after pretreatment in nitrogen exhibited higher catalytic activity for NH3 decomposition comparing to that observed after hydrogen pretreatment of the sample. Cobalt dispersion in MWCNTs-supported catalysts was found to be a function of thermal pretreatment temperatures. The studied samples were characterized by X-ray diffraction, HRTEM, thermo gravimetric analysis, temperature programmed reduction and temperature programmed desorption (C) 2013 Elsevier B.V. All rights reserved.