The influence of grain size on the catalytic activity of Ni-based solid-state catalysts in the thermocatalytic decomposition of methanol was investigated. The carbon deposit, obtained during the catalytic activity and stability tests, was analyzed in detail by scanning electron microscopy (SEM), Brunauer–Emmett–Teller analysis (BET), and X-ray diffraction (XRD). It was found that the Ni₃Al catalyst with a bigger grain diameter exhibits higher catalytic activity and stability in a methanol decomposition reaction. The reason for the differences in the catalytic activity and stability of solid-state catalysts depending on the grain diameter of the catalyst was proposed. At the tops of the obtained nanotubes/nanofibres, one can see Ni nanoparticles in all investigated Ni₃Al thin foils with every tested grain size.