Ni-carbon nanofibers (Ni-CNFs) catalysts were synthesized in situ by the decomposition of carbon-containing gases (i.e., CH4, CO, and C2H4) over Ni/Al2O3 catalyst and directly used to catalyze ammonia decomposition. The results showed that Ni nanoparticles were found to locate at the tips of CNFs when using CH4 and CO as carbon sources, while they located at the roots of CNFs when using C2H4 as a carbon source. For ammonia decomposition, Ni catalysts at the tips of CNFs showed higher activity, which could be due to the more accessible surfaces to the reactants. Interestingly, the Ni catalyst at the tips of CNFs with CH4 as a carbon source exhibited higher activity than that with CO as a carbon source, even though the former catalyst had a larger average particle size. The possible mechanism was given by combining characterization results with our previous simulation results. Finally, when using CH4 as a carbon source, the effect of the Ni-CNFs catalysts with different growth times on the activity was further studied.