The deactivation of cobalt based catalysts in slurry Fischer-Tropsch reactor has been modeled assuming a sintering mechanism which involves the intermediate formation of cobalt oxide layer on metallic nanoparticles. The mechanism, correlating the crystallite size growth to the water to hydrogen concentration ratio in the liquid phase, has been used to describe the activity decline with time on stream. The effect of operating conditions on the rate of sintering is considered. It is found that at the same initial conversion, sintering rate is higher for lower H-2/CO ratios, whereas higher ratios could lead to larger crystallites once operated at constant gas flow rate. The presence of water in the inlet syngas stream also accelerates sintering. The sintering model is then used to describe the deactivation in laboratory-scale slurry reactor. (C) 2013 Elsevier B.V. All rights reserved.