The effects of inlet gas velocity and reactor size on methane conversion and carbon yield of methane decomposition on a Ni-Cu-Al catalyst in a fluidized-bed reactor was investigated in this research. Phenomenological modelling and simulation of the reactor were performed to achieve the objective. The axisymmetric 2D model of the reactor comprises gas- and solid-phase mass balances, energy balance and momentum balance. Axial and radial dispersion concepts in gas and solid phases are applied on the description of the non-ideal flow pattern in the reactor. Solid-phase dispersion takes into account aggregation as the result of the carbon nanotubes growth on the catalyst surface. Simulation results exhibit that when the inlet velocity escalates from 0.017?m/s to 0.030?m/s, the methane conversion rises from 77% to 90%, and the carbon nanotube yield increases from 0.68?g/g catalyst to 1.5?g/g catalyst. The methane conversion in the reactor with the diameter of 0.14?m is 49% and increases to 77% when the diameter enlarges twice with the constant aspect ratio of 2.82. The carbon nanotube yield decreases from 0.86?g/g catalyst to 0.66?g/g catalyst in 5-hour operation with the increase in the size.