Two Ni/Al2O3 samples prepared using incipient wetness impregnations with and without argon glow discharge plasma treatment were tested for methane decomposition to H-2 and carbon nanofibers. The plasma treatment induces a significant change in properties of catalytic methane decomposition. The plasma-treated sample shows a low CH4 decomposition rate and low final carbon yield, as a result of a higher concentration of a close-packed plane in the Ni particle, smaller Ni particle sizes, and stronger Ni-Al2O3 interactions. The carbon nanofiber growth pathway shifts from a mixture of tip and base growth for the nonplasma-treated sample to base growth for the plasma-treated sample. Very different from the carbon nanofibers obtained over the Ni catalyst prepared without plasma treatment, the tips of about 80% carbon nanofibers produced over the plasma-treated sample are open.