The experimental data, corroborating the effect of CH4-H-2 mixture compositions and temperature on the formation of catalytic filamentary carbon (CFC) in methane decomposition over a highly loaded Ni-containing catalyst, are presented. We propose a model of the catalyst deactivation : carbon blocks the surface of a CFC growth center (GC). Using this model, we have derived the equation for calculating the deposition rate of CFC and its maximum quantities which may form during the time of complete catalyst deactivation.